Hi Christian,
Happy Finally 
But the question is - "guest" account is always available in the sybase ASE Database may be chances that it got locked or anything. Infact that was my next question to ask you the output for
select * from master..sysusers
and
select * from master..syslogins
Anyways lets dig if you get the problem again
Now this thread deserves to get marked and closed so that our fellow DBAs can go though if they get this kind of problems
Cheers
Kiran K Adharapuram