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Winforms ShowDialog(this) blocks the switching windows of other applications to the foreground

When I open any dialog in my Winforms application then Windows 10 behaves oddly in these ways:
ALT-TAB no longer displays the list of windows
I cannot switch to hidden applications using taskbar icons - if I select a taskbar icon of a window that is not currently visible (even if it is not hidden by the winforms app) the icon just flashes and then stays highlighted, but is not brought to the foreground. I can use other applications that are visible. As soon as I close the dialog form the other I can use the windows of other applications correctly
If I switch to the application window that is behind the winforms application by clicking on it, I cannot go back to the winforms app either by ALT-TAB or by clicking on the taskbar icon. I can get back to it by minimizing the other application
I'm opening the dialogs with
dialogFormName.ShowDialog(this);
TopMost is set false on all forms and is not set in the code.
I've read about 50 related articles and the only problems seem to be either TopMost is set or ShowDialog is not called with the parent form. I'm not a regular Winforms developer so I'm probably doing something stupid. This is driving me crazy, so I'd really appreciate any help!
Edit: The same issues occur with MessageBox.Show(this, "test"). The issue does not occur with a newly created app just with a single button calling MessageBox.Show(this, "test"). The problem application uses EntityFramework but no other packages and the problem existed before I added EF.

After trying different scenarios I eventually found the issue. In my case I was calling ShowDialog() after a user clicks an item on a ContextMenu. The blocking of ALT-TAB was caused by the following code that attached the ContextMenu to the ListView that the menu was contextually for:
lstList.ContextMenu = myContextMenu;
As soon as I removed that association, the ShowDialog no longer blocked ALT-TAB.

Form.ShowDialog() blocks the parent form until it's closed.
Use Show() to display the form separately without blocking its parent.

How to get the topmost window if it is modal and inactive

I want to prevent users to run my application multiple times on the same machine so I used a solution from this thread: What is the correct way to create a single-instance application?
This works OK, but I have a problem displaying my application when a modal window is opened (for example with view.ShowDialog();). This is a scenario:
User runs my application and opens a modal window.
Then he tries to run my application again, the code in the startup procedure of this second instance of the application finds another application running and broadcasts a WM_SHOWME message to it to show it self. Then the second instance of the application terminates.
The first application receives the WM_SHOWME message (using the solution from How to handle WndProc messages in WPF?). Now it should bring the topmost window to front, and this is my question - how can I get the topmost window of my application if the topmost window is modal and not even active? I tried with the solution from Refer to active Window in WPF? but of course my windows aren't active so this doesn't work.
PS - when the application is running and a modal window is opened and when I hover over the icon in the task bar, then I can see two windows - main window and a modal window. I can click on the main window (which is of course disabled because a modal window is on top of it) and I can click on the modal window also. My solution works just like if I would click on the main window, but I want it be able to activate the topmost window, which is modal in this case.
So, any idea how to bring the topmost modal window (or main window if no modal windows are shown) to the front?

The behavior as described in the question indicates that the main window is not owning the dialog.
Note that when a dialog is owned by a (main) window, then the window cannot cover the dialog (the dialog will normally always stay on top of the window).
This also has the effect that when bringing the window to the front, the dialog will also be brought to the front on top of the window -- which neatly will solve the problem you have.
Setting the owner for your dialog (modal window) is rather easy. Simply set its Owner property to you main window before showing the dialog, similar to this example:
Window modalWindow = ... create modal window instance
modalWindow.Owner = mainWindow;
modalWindow.ShowDialog();
(Side note: If it is also desired to have only the icon/thumbnail of the main window appear in the task bar, then the ShowInTaskbar property of the modal window should be set to false.)

The best solution to make your application a single instance on a give machine is to use Named Mutex
Mutex
Here's the excerpt from the same documentation
Mutexes are of two types: local mutexes, which are unnamed, and named system mutexes. A local mutex exists only within your process. It can be used by any thread in your process that has a reference to the Mutex object that represents the mutex. Each unnamed Mutex object represents a separate local mutex.
Named system mutexes are visible throughout the operating system, and can be used to synchronize the activities of processes.
You can create a Mutex object that represents a named system mutex by using a constructor that accepts a name. The operating-system object can be created at the same time, or it can exist before the creation of the Mutex object. You can create multiple Mutex objects that represent the same named system mutex, and you can use the OpenExisting method to open an existing named system mutex.
And, anyways you've handled the case where you want to bring the first instance forward.

Showing form when main form gets closed

I have tried around with threads a little bit but I can't figure out the right way to get what I want (because I'm not very familiar with threads).
I have a "Sign out" button in my main form. When clicked then the Form shuts completely down and restarts (Application.Restart()). Now I wanted another form pop up when the main form gets closed (Just a label with "Loging out - Please wait"). When the main form opens then this "Log out form" should disappear.
I know I should make the Sign out form as foreground thread but how do I call the Form.Open() method with the thread and how I can get it closed when the main form starts?

Try to show the second form in the FormClosing event of the main form.
When the main form closes it most certainly "shuts down" the message loop of the application making it impossible to spawn any new window or closing any window that is already open.
Edit:
As of the comments: Showing the logout form from the process that is about to Restart until the new process has been started successfully will not be possible because the old process will be shut down and therefore any open form it shows.
Possible solutions:
Write a program with the solely purpose of showing the logout form and start that before restarting the application. After the Application has startet again search the process of the logout form and stop/kill it.
Avoid to have to restart your application completely

Close a WPF window separately

Goal:
Enable closing of the application's window(s) independently without affecting others. The application is created in WPF.
Problem:
Can't close the window(s)
In winform, it is enough to have the code winform.close() to close down the window but it doesn't work in WPF.
You can have this code to close a specfic window:
Application.Current.Windows[0].Close();
but how would it work if you have many windows and you want to close a specific window without affecting the others?

Use the Application class to get the windows through Application.Windows-property exactly as you described. If you are in the code-behind of the window, call this.Close();
Configuration for multiple Windows
Set the main window to the Application.MainWindow property and set the Application.ShutdownMode to a appropriate value if you also want to hold the app open, if the main window is closed (e.g App.Current.ShutdownMode=ShutdownMode.OnExplicitShutdown; ).
I have already observed, that some people have had problems with the ShutdownMode. A workaround for this is to open the first window invisible and from this window, you open the visible application windows. This prevents the application from closing if the first created window will be closed. However you should be able to resolve this problem also over the ShutdownMode-property.
In scenarios with multiple windows, you can use Shutdown to close the app without closing every window.
I hope this answer is what your question is about. Make a comment if not.

I am agree with HCL. You can use this.Close(); from code-behind of the window, this will close WPF window as like winform.close();.
Or you can use following code for get the specific window for close
Window win = Application.Current.Windows.OfType<Window>().SingleOrDefault(w => w.Name == "Window Name");
win.Close();

just use this code to close the most recent window
Application.Current.Windows[Application.Current.Windows.Count - 1].Close();

Application disappears in the background

in my application there is a print form. When you click the print button, the windows print dialog appears, showing the printed page.
When it's finished the print form is closed (this is still ok) but my application is set to the background on the z-order (this is not ok) and another application window is set to the foreground.
I could help myself by calling WinAPI-SetForegroundWindow() from my application, but it flickers and I don't think that this is a clean approach.
A clean approach would be that my application doesn't get set to the background.
Any suggestions?

try to call .ShowDialog with your form as the first parameter. that should bring your form into the front after the dialog is closed.