Source: Onepoint Operations
Event ID: 25213
Desc: An unknown token name (sc-bytes) was encountered.
Source: Internet Information Server web server log.
Description: IIS Application Log - Web Server. File: ex021203.log

Contributed By: Ugo Corti
You will see this event when there is a partially written IIS event to the IIS log. This is a bug with the MOM agent.

From the microsoft.public.mom newsgroup
MOM also has problems parsing the W3C Extended Log format. If you are using this format try switching to the IIS format. Specifically, there is a problem with the extended properties, so you can't use any of them. However, this doesn't leave you with much (in fact, I have yet to hear of anyone getting extended logging to work even without the extended properties).