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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

Server error in '/' Application .asp webforms

I've added to my .asp website a CreateUserWizard with destination page url home. After I'm launching it in Visual Studio 2013, it opens, I register, but, after a long delay after pressing sign up, it shows me this 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)
Should I create a database myself or how it can't locate the server?

I think that connection string for database is not correct. Try creating an empty database and add that connection string in web config.

The error is telling you it can't find the database you have specified in your connection string (perhaps a typo or an incorrect details)
this page will help you create the connection string. You will just need your server details

ASP.NET MVC failure to create database [duplicate]

This question already has answers here:
Cannot Connect to Server - A network-related or instance-specific error
(54 answers)
Closed 6 years ago.
I'm new to ASP.NET MVC and was just messing around with it and trying to see how it works. So I decided to make a query to the database accessed with the default ApplicationDbContext. Normally, if the database hadn't been instantiated before that, this query would make sure to first create the database and then make the query, right? Well, here's what I did:
// This is the default Index action in the Home controller:
public ActionResult Index()
{
var db = new ApplicationDbContext();
var usersCount = db.Users.Count();
return View();
}
I know that the query is pretty meaningless but the sole purpose of it was for it to generate the database. The thing is it that it doesn't. I get a System.Data.SqlClient.SqlException when trying to execute the query (second line of the method).
Even when checking the App_Data folder I see that the database isn't created. What could be the cause of this? Strange thing is that this works on my desktop computer, but not on my laptop. But I did notice that when I put an invalid Data Source(ex. Data Source=.) in the connection string I get the same exception on my desktop computer. So maybe something is happening with the LocalDb that's causing this behaviour.
The connection string:
<connectionStrings>
<add name="DefaultConnection" connectionString="Data Source=(LocalDb)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\aspnet-DefaultProject-20160731061747.mdf;Initial Catalog=aspnet-DefaultProject-20160731061747;Integrated Security=True" providerName="System.Data.SqlClient" />
</connectionStrings>
This is the full exception:
An exception of type 'System.Data.SqlClient.SqlException' occurred in EntityFramework.dll but was not handled in user code
Additional information: 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: 52 - Unable to locate a Local Database Runtime installation. Verify that SQL Server Express is properly installed and that the Local Database Runtime feature is enabled.)
I would be very grateful if someone can come up with a solution for this!
P.S: Everything works fine on my laptop if I change the Data Source to my SQL Server Express server name.

Sounds like your LocalDb instance isn't installed. This is a bit of a "dev attachment" version of SQL that's installed with certain versions of Visual Studio and might have been de-selected or just plain not included perhaps if the original install was older and has gone through upgrades, etc.
This post gives a decent explaination: How to install localdb separately?
Basically google for SqlLocalDb.msi and follow the instructions.
I generally prefer Express over localDb anyway, but that's up to you.
Also note that connection strings are considered "loose" information in the web.config for MVC stuff. That is, they tend to be specific to the machine, so if you're changing machines (or working on a team), your machine is likely to have a different connection string (or a local copy/partial of web.config).

Can't open connection to SQL server

When I try to connect to the MS SQL server in the local domain with SQL with SQL server authentication using the following code fails:
SqlConnection sql = new SqlConnection(conString);
sql.Open();
sql.Close();
I can connect to my local SQLExpress test database just fine, using either windows or sql authentication. I tried using many different connections strings for the domain database, including using the connection string generated by (successfully) adding the server as a DataSource. What am I doing wrong?
EDIT: When I add the server in the server explorer of visual studio, I can connect successfully. I would assume that the connection string thereby generated is valid:
Data Source=mySubDomain.myDomain.local;Initial Catalog=myDatabase;Persist Security Info=True;User ID=myUser;Password=myPassword;
I checked and remote connections are allowed. I do not have permission to access the windows server which the sql server is running on, so I can't check any further settings.
EDIT 2: The following message comes with the exception:
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)
EDIT 3:
I tested my application on a different computer in the same domain, and it worked. Does someone have a clue what's going on or how I could find out?

The Group Policy or firewall (or both, not sure) of this company domain make my .exe file (and any other .exe) behave in the following way:
Launching it from C:\Program Files (or C:\Program Files (x86)) as any user works fine
Launching it from anywhere else as a normal user is not allowed and fails
Launching it from anywhere else as a local admin will execute the application, but connections are blocked

"Can not find server or is not available" on test server but not while debugging

I keep getting an error when trying to access the database from a web site it returns an error that it can't find the server or it's inaccessible.
Now I did a search and I found many possible reasons - I turned on named pipes and also did what this link suggested (i.e. set the IP addresses to "enabled") yet I'm still getting the error.
Error SqlException: System.Data.SqlClient.SqlException (0x80131904):
Error An error has occurred with the network or the occurrence when
connecting to SQL Server. Can not find server or is not available.
Verify that the instance name is correct and that the configuration of
SQL Server allow remote connections. (provider: Named Pipes Provider,
error: 40 - Could not open a connection to SQL Server) at
System.Data.SqlClient.SqlInternalConnection.OnError(SqlException
exception, Boolean breakConnection) at
I also downloaded this little app and I copy/pasted the connection string from the web.config used in IIS for this site and the app is able to connect and query the same database being targeted by the web site.
And, as the title says, this error is utterly missing when I debug it in VS2010. What could be causing this error when launching from the server?
Edit: the connection string includes a user name and password (it's not Windows Authentication).

Most likely a security issue. If using windows authentication then make sure the thread pool that the web application is running as has sufficient DB access\privileges.

The issue is that the web.config references another config file
<appSettings file="config/dev.config">
and so the connection string in web.config is ignored over the one in dev.config - an error to have it in both (the one on the test server was created by someone else and worked on static pages - it only failed when the website had to query the database), that's why the version in Visual Studio was running fine.