Source MSFTPSVC Event ID 8: FTP Server could not create a client worker Thread for user at host xxx.xxx.xxx.xxx. The connection to this user is terminated the data is the error. (No error Code)

Where xxx.xxx.xxx.xxx is the host attempting to connect.

The MS KB article here http://support.microsoft.com/?id=293637 proposes a solution with regards to a related metabase error and how to delete such. However some users may not have the "ipsecurity" error described in the KB, so instead you will have to check each of you individual FTP sites. Make sure that in the Properties -> Directory Security tab, Granted Access is selected. If in any case Denied Access is selected, make sure the connecting host above is added in the exception list.

After making changes to the Directory Security property, double check with the KB article again as changes to the metabase have been made.

After the two checks above and you are still getting tons of Event ID 8, you can also check here http://technet.microsoft.com/en-us/library/cc783062%28WS.10%29.aspx