samba - Mar 2004 - Samba 3.0.2 an Win2k3 AD DC

I try to integrate a Samba/CUPS Printserver into Windows 2003 Active
Directory. First it works fine. I could join the domain, the users could
print an access the print shares.
But after a couple of hours (10-12 I think) I get errors about invalid
credentials. I think Samba didn't renew the Kerberos Tickets.

Used Software
SuSE 9.0, Samba 3.0.2a, heimdal 0.6

Thanks for any help

Marcel Volz