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IIS seems to not being able to connect using Entity framework provider

I've being trying in vain to solve a problem on a newly created windows server with IIS hosting an ASP.NET application that was copied from another environment. The issue is that I cannot manage to connect to the database when the application tries to open a connection via entity framework.
If this is off topic for SO let me know so I can move to another channel, I thought it belonged here as seems specific to EF.
In detail, I have the following connection strings:
<connectionStrings>
<add name="ApplicationServices" connectionString="Data Source=LOCALCOMPUTER\SQLEXPRESS;Initial Catalog=xxx;Persist Security Info=True;User ID=yyy;Password=zzz" providerName="System.Data.SqlClient"/>
<add name="MembershipConnection" connectionString="metadata=res://*/App_Code.Membership.Membership.csdl|res://*/App_Code.Membership.Membership.ssdl|res://*/App_Code.Membership.Membership.msl;provider=System.Data.SqlClient;provider connection string="Data Source=LOCALCOMPUTER\SQLEXPRESS;Initial Catalog=xxx;Persist Security Info=True;User ID=yyy;Password=zzz;MultipleActiveResultSets=True;Application Name=EntityFramework"" providerName="System.Data.EntityClient"/>
...
xxx, yyy, zzz of course have the proper values. LOCALCOMPUTER is the FDQN of the local computer, which is not on a domain
The first connection string is for the Microsoft login library, while the second uses entity framework from our code.
When I browse to the site, I can successfully access the login form, and if I try to insert a wrong password i get a message saying so, meaning the application managed to check the database using the first connection string, but when i insert a correct one, i land on an exception as soon as the code tries to create a context.
The underlying provider failed on Open.:
A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: TCP Provider, error: 0 - The wait operation timed out.)
In the event viewer it tells me that the user has been authenticated via Authentication Type: Forms , which I assume it means the auth system (the one using ApplicationServices connection string) is working, the stack trace confirms that the error occours when I try to use EF in code, meaning the DB has been used succesfully to check user credentials
note that:
The web app works in other environments
I can connect to the sql server using the credentials via SSMS
I've assigned a local windows account to the app pool that can also access the database, and tried changing to Integrated Security=SSPI
Both .Net 3.5 and .Net 4.8 are installed
The database is on the local machine and has been restored from a dump of the other one, its users have been mapped, and to rule out issues i gave it ample authorisations with dbowner datawriter and datareader
I've been trying to fix this since a couple days checking similar issues on SO with no avail and I don't really know how to proceed, is there something I can do to troubleshoot the issue?

Apologies, I've found out the answer, and it's specific to the code base. Apparently the connection string was being ignored (despite being perfectly usable) in favour of data saved in a table.
Had to contact the previous developer to find out, guess it's a good example of how important is to document your code
I'll close the thread, thanks for all who tried to help

Service-Based Database Entity Framework Connection Error On released version

I am writing a Windows Forms application, where I want to save data inside the application. For the reason I used a serviced-based database with a .mdf database file, and I use Entity Framework.
So far everything is working fine until I released the file and opened it on another computer. The application opened and everything worked fine but when it comes to the database interaction, it throws a big error like this:
************ Exception Text ************
System.Data.Entity.Core.EntityException: The underlying provider failed on Open.
System.Data.SqlClient.SqlException: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 50 - Local Database Runtime error occurred. The specified LocalDB instance does not exist.)
Here a screenshot of the error: https://www.dropbox.com/s/gwban6ab97c6fya/22093543_10213995193571435_760919436_n.png?dl=0
In case you need the project its uploaded to here: https://www.dropbox.com/s/ubpc683ggtihh6k/usercontrol.zip?dl=0
I have tried in so many ways but nothing is working, same thing happens for the installer version as well.
Can anyone help me on this please?
Here are my connection strings:
<add name="DatabaseEntities"
connectionString="metadata=res://*/dbModel.csdl|res://*/dbModel.ssdl|res://*/dbModel.msl;provider=System.Data.SqlClient;provider connection string="data source=(LocalDB)\MSSQLLocalDB;attachdbfilename=|DataDirectory|\Database.mdf;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework""
providerName="System.Data.EntityClient" />
<add name="OPLCheque.Properties.Settings.DatabaseConnectionString"
connectionString="Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated Security=True"
providerName="System.Data.SqlClient" />

I'm pretty sure this post would be flagged as duplicate since a similar question has been answered multiple times already, but here you go.
You are using LocalDB which means it cannot be shared since it runs on SQL Server Express.
If you simply want a LocalDB, see this answer to implement it after publishing your WinForms app.
Basically you want to publish your app with prerequisite components so you should publish those along your application.
If you want to use a single database to be shared between instances of your application, learn how to setup one using SQL Server here.
Afterwards, learn what connection string format you should use.
Sample to connect via IP Address:
connetionString="Data Source=IP_ADDRESS,PORT;
Network Library=DBMSSOCN;Initial Catalog=DatabaseName;
User ID=UserName;Password=Password"
See this for more.
If ever you plan on using a shared database, better make sure that the connection string that your database context should use is specified.
Config:
<add name="DefaultConnection" connectionString="" providerName="" />
Database Context:
public DbContext()
: base ("DefaultConnection")
{
}

cannot deploy c# app with entity framework

i have finished a desktop c# project in which i make use of Entity framework 5 and SQL server 2008
in my machine it's working well, but i created a virtual machine and tried to deploy it there and I have problems.
I get this error : System.Data.EntityException: The underlying provider failed on Open. --->
System.Data.SqlClient.SqlException: A network-related or instance-specific error occurred while establishing a connection to SQL Server.
The server was not found or was not accessible.
Verify that the instance name is correct and that SQL Server is configured to allow remote connections.
I used installshield for the setup and I have installed sql express in the visrual machine, during the setup I create the database and all the tables.
my connection string is this :name="BussinessContainer" connectionString="metadata=.\Bussiness.csdl|.\Bussiness.ssdl|.\Bussiness.msl;provider=System.Data.SqlClient;provider connection string="data source=localhost;initial catalog=yingyangDB;User ID=username;Password=password;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework"" providerName="System.Data.EntityClient"
Should I change the connection string?
I searched a lot before posting this, but nothing worked..any help?

SQL Express (normally) doesn't create as a "Default instance" - i.e. connecting by localhost isn't sufficient to connect to the local SQL express instance.
You may need to use localhost\SQLExpress to connect to the local SQL Express instance (the instance name may be different, but usually it is not.
Additionally, you specify an Initial Catalog in your connection string, which I assume is pointing at your new DB. However you say that you create the DB during the installation - so that DB may not actually exist in your Instance yet. That may be causing it to fail out. Try taking it out of the string and see if that lets you connect.

app.config and Entity Framework for remote connection

I have a connection string to a local database which works ok.
<connectionStrings>
<add name="Entities"
connectionString="metadata=res://*/RNADataModel.csdl|
res://*/RNADataModel.ssdl|
res://*/RNADataModel.msl;
provider=System.Data.SqlClient;
provider connection string="
Data Source=my-localhost;
Initial Catalog=my-db-name;
Integrated Security=True;
multipleactiveresultsets=True;
App=EntityFramework""
providerName="System.Data.EntityClient"/>
</connectionStrings>
However I cannot figure how to connect to a remote database succesfully.
I have tried to add User ID=user; Password=pass; and also disabled Integrated Security but the server fails to connect.
Is there a way to fix the connection string or some other workaround to connect to the remote database?
EDIT:
The error comes from my web app every time the connection string is not correctly specified:
Server Error in '/' Application.
The server was unable to process the request due to an internal error. For more information about the error, either turn on IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute or from the <serviceDebug> configuration behavior) on the server in order to send the exception information back to the client, or turn on tracing as per the Microsoft .NET Framework 3.0 SDK documentation and inspect the server trace logs.
and
System.Data.EntityException throws: {"The underlying provider failed on Open."}
If I use SQL Server management studio I can see the database on the server and credentials works fine. Might be some firewall configuration, any suggestions?

You need to change Data Source=my-localhost to
Data Source=ServerName\InstanceName
Don't forget to specify the correct UserName & Password

I managed to access the customer's server, so following this guide http://blogs.msdn.com/b/walzenbach/archive/2010/04/14/how-to-enable-remote-connections-in-sql-server-2008.aspx
I have enabled the remote connection. The Connection string was fine. Thanks the comments

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found,etc

I'm just publishing my website to my domain name and when I try to access it in my browser I get the Following error:
A network-related or instance-specific error occurred while
establishing a connection to SQL Server. The server was not found or
was not accessible. Verify that the instance name is correct and that
SQL Server is configured to allow remote connections. (provider: SQL
Network Interfaces, error: 26 - Error Locating Server/Instance
Specified)
I think the problem is into my connection String ==>
<configuration>
<connectionStrings>
<add name="ApplicationServices" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|aspnetdb.mdf;User Instance=true" providerName="System.Data.SqlClient" />
<add name="TennisOnlineContext" connectionString="Data Source=|DataDirectory|Tennis.sdf" providerName="System.Data.SqlServerCe.4.0 " />
</connectionStrings>
...
</configuration>
Sorry but it's my first web site where I try to publish... So, please what I'm doing wrong ? Thanks in advance...

Have you tried localhost\SQLEXPRESS? Ensure all of your connection keywords abide by those outlined by Microsoft here.
You can check if a connection string is valid, or indeed build a valid connection string that you can copy, by using the SQL/Database Connection utility that ships with VS2010. In VS2010 goto Tools, Connect to database. This will launch the connection dialog; here you can build and test connection strings. For advance connection features, once in the dialog click Advanced. From the advance dialog you can copy and paste connection strings.
Microsoft have also released this dialog to be freely used and distributed in .NET applications. If you interested in this it can be found here.
I hope this helps.

If you are connecting from Windows machine A to Windows machine B (server with SQL Server installed), and are getting this error, you need to do the following:
On machine B:
1.) turn on the Windows service called "SQL Server Browser" and start the service
2.) in the Windows firewall, enable incoming port UDP 1434 (in case SQL Server Management Studio on machine A is connecting or a program on machine A is connecting)
3.) in the Windows firewall, enable incoming port TCP 1433 (in case there is a telnet connection)
4.) in SQL Server Configuration Manager, enable TCP/IP protocol for port 1433

If you look into your release version of web.config you will find <roleManager enabled="false"
Change this configuration setting to True
You can find details here at this link below
http://dotnet1blogger.blogspot.com/2012/04/network-related-or-instance-specific.html

please check once TCP/IP properties in SQL server configuration manager
for more details here

Enable TCP/IP and Named Pipes in SQL Server Configuration Manager.
From SQL Server Configuration Manager, select SQL Server Network Configuration, then select the appropriate instance (if you have multiple). Right click and select enable for Named Pipes and TCP/IP.