Could you let us know more about the setup?
Is it by any chance that you installed SP2 on the client machine and not in the server you connect to? Is the client Vista or 2008? or are you using XP/2003 as clients?
I think there might have been a compatibility issue introduced between client SP2 talking to a Server SP1 which will cause this.
For now the only thing I can think of is instally SP2 in the server should solve the problem.