Under 3.0.21a i got this message from a log of a specific windows XP pc :
[2006/03/07 15:30:48, 0] rpc_server/srv_netlog_nt.c:_net_sam_logon(667)
_net_sam_logon: creds_server_step failed. Rejecting auth request from client PC-STEMILION machine account PC-STEMILION$
I reinsert the pc to the domain and it appeared that users could logon.
After a while users are not able to logon again.
I upgraded to 3.0.21c, the error message vanished, but the problem persists.
This is something I believe we've fixed for 3.0.22, but unfortunately the change was too large for 3.0.21b. Once 3.0.22 is in pre-release I'd appreciate it if you could try out the new code.
Please reopen if the bug still exists on the SAMBA_3_0 tree.