I just had the same problem, where the userid seems to be invalid.  Lucky for me, my Forum is brand new, so deleting the group wasn't painful.  When it happens again, it will be much more ugly.

Not sure why no one has replied to your messges.  Maybe a second report will prompt a reply.

Hi all.

I have this problem in my forum. I have readed that I have to change the UserID 0 to other valid value, but I don't know what is the table on bbdd. if I can change from web of user that created the post I think that the problem will fix. is it posible?

thank you very much

Run this script in Host - SQL with "run as script" enabled.  If you know the userID of the user you can change it in this script.  This script sets all posts with your problem to a annonymous user.

UPDATE {databaseOwner}{objectQualifier}Forum_Posts
SET UserID = -1
WHERE UserID = 0

Go