SGE 5.50 DBAuth failed

I have another Problem, hope someone could help me...

...when I open http://<server>/sgnsrv/ in the browser, and klick on "CheckConnection", and then "invoke", the following Error appears:

 <?xml version="1.0" encoding="utf-8" ?> 
  <string mlns="http://tempuri.org/"><Dataroot><WebService>OK</WebService> <DBAuth>failed</DBAuth> <Error>Datenbank-Ausnahme aufgetreten.</Error></string> 

I searched in the KB, and found this: 

http://www.sophos.com/support/knowledgebase/article/107848.html 

and this:

http://www.sophos.com/support/knowledgebase/article/108339.html

I do what in the PDF is described, but that did not solve the Problem.

I'm using Windows AD, a user is created and added to the SQL Server, correct FS permissions are set, and management config packet is also correct. I changed the identity of the DefaultAppPool in IIS to the DB-User, but the failure appears  nevertheless

Any suggestions?