Make it log id,user name, maybe kills/deaths et c. to see if the player whose authid is blank is actually connected.
There's a chance WC3FT is calling functions on non-connected players.
Not like I have to say it but ... this could be some serious overkill in its logging, so you might like to only get a few ;-)
The other option is to make the plugin announce this error to any Admins connected.
Then they can check if the player is connected ( status returns player id's, right? )