Windows 2000 KDC - pre-authentication failed

[Someone]

Hello,

I am using a Windows 2000 KDC (running AD and all that microsoft stuff) 
and have UNIX clients using MIT kerberos5 1.2.3 which authenticate 
themselves to that w2k KDC. It works not to bad, well at least I can 
login with the password which is on the windows domain controller but 
what annoys me is that in the event viewer (section security) of windows 
2000 I see the following error:

Pre-authentication failed:
  	User Name:		username
  	User ID:		DOMAIN\username
  	Service Name:		krbtgt/REALM
  	Pre-Authentication Type:	0x0
  	Failure Code:		0x19
  	Client Address:		X.X.X.X

Does anyone have an idea what that could be ?

Regards