Someone else reported a similar issue via email. In his case, the XNA window apparently didn't get all WM_KEYUP messages (as far as I can tell, AltGr is Ctrl + Alt combined), so the GUI assumed the user was holding down the Ctrl key. Pressing either
of the Ctrl keys on the keyboard normalized the situation again. Is the behavior the same in your case?
We didn't have time to debug the issue properly and his users didn't need to enter any '@' characters, so he simply filtered the control key via an adapter class between the GuiManager and the IInputCapturer.
I'm having trouble reproducing the issue because I'm using an English keyboard. If you have the time, could you run Spy++ (comes with the full Visual Studio editions) or ManagedSpy (available from Microsoft, couldn't get it to run on my system) to
capture the window messages sent to the XNA game window when you press and release the AltGr key?
I could then simulate the key on my system by sending those messages manually ;-)