ALM-Client does not work anymore (ALM-Client funktioniert nicht mehr)

We have a wired error message when starting ALM-Client on 3 PC's

These PC worked fine yesterday but not anymore today????

 

It appears after  Staring ALM (launching the browser (we use IE7) but before the login Screen is displayed ALM-Client does not work anymore (ALM-Client funktioniert nicht mehr)

If you click details it says: An unhandled exception ('System.Configuration.ConfigurationErrorsException') occurred in ALM-Cleint.exe.

 

IE freezes and you have to kill it though task manager.

 

We have ALM 11 Patch level 4 or SP2,  Win 2008 R2 Server and MS-SQL DB (original english versions)