Contextless login fails: Novell Client 2 SP2 IR4a on Win7

Hello,
I am running about 200 Windows XP SP3 machines with Novell Client 4.91 SP5. Contextless login works always fine.
I am running also about 40 Windows 7 SP1 machines with Novell Client 2 SP 2 for Windows IR4a. I use the same settings for both Novell Clients.
Randomly the contextless login fails on Windows 7. The error message says, that the user doesn´exist. It works when the user browse the tree via the "Context" button.

Platform:
OES 2.0.3
DS Version: 20602.00

Any suggestions how to fix this problems are very welcome.

Kind regards
Karin