I've got a funny problem connecting to an AOS server.

I got a domain network containing a (VM) Server running the Application Object Server (AOS).

Client computer A) can connect to the AOS without problems and work.

When client computer B) tries to connect, all I get is this error message: Logon Error
Connection with the Application Object Server could not be established.

The event log of Client B) does not contain any messages about this.
Firewalls are off on all three computers (by GP).
Tcp/ip from cliebt B) to the AOS server does run fine.
Both clients run the same OS (win 7 RC), sit on the same subnet, next to each other. Yesterday it all worked, today just one computer can connect.

Any ideas what might cause this problem, how to resolve it, or how to debug it?