Event ID: 4 messages after upgrade to IAS on 2003

Hello,

The upgrade from IAS on 2000 to 2003 was successful, everything works as
expected. The problem is there are many messages (four at a time) in the
system log with the following information:
----------
Accounting request for user smith was discarded.
Fully-Qualified-User-Name = <undetermined>
NAS-IP-Address = 1.1.1.1
NAS-Identifier = <not present>
Called-Station-Identifier = 5
Calling-Station-Identifier = 555-1212
Client-Friendly-Name = RASSERVER
Client-IP-Address = 1.1.1.1
NAS-Port-Type = ISDN Sync
NAS-Port = 20010
Proxy-Policy-Name = Use Windows authentication for all users
Accounting-Provider = <undetermined>
Accounting-Server = <undetermined>
Reason-Code = 1
Reason = An internal error occurred.
----------

What causes the messages and how can I eliminate them.

Thanks