why a user can connect to the web only after iisreset?

The user couldn't connect to the web and get always the exception:

"Beim Ausführen der Formularmethode F0_ctl00_Main_Main_ControlRef1_Button1_Method ist ein Fehler aufgetreten. Object reference not set to an instance of an object" 

and via WebDesigner I can only sign in after "iisreset" otherwise I get the message "User ist nicht berechtigt für das Web"

This is in Version 8.1.3. I installed the AppServer and the WebServer without ssl.

Did anyone know where I can look for the exception?

Regards

Friedhelm