AX user session is no longer valid


we have suddenly the message “Your Microsoft Dynamics AX user session is no longer valid. Logoff your computer and logon again. If the problem persists, conact your Microsoft dynamics administrator”.

I have 4 AOS servers running. On only one AOS server I have this problem. On this server I have 3 AOS instances running and on all 3 I have the same problem.

We are running AX4.0 SP2 on windows 2003 R2 SP2 server.

anyone any idee?