Event ID 1053 - Userenv

I am having ISA Server 2004 running on Windows 2003 Server with Service Pack 1. Right now, everthing is working fine at the moment, but the following event called ID No. 1053 is continuously showing up in the Event Viewer. Windows is not able to determine the user or computer name because the remote procedure call failed and did not execute. The group policy processing also aborts. Is there anyone with some idea if this is ISA Server related or something else? Thanks.