I can't point you to an exact answer but we had a problem with
this module a while back and it was failing because a field was missing in users table or copy/view thereof.... and it turned out that it was a field we had added for OTHER purposes. At the time, I thought that my adding a field for OTHER purposes shouldn't break the spam module...
This module, i think uses a view or a copy of the original table and and.... check your error log and follow the dusty trail 'till you find the bug!
-Bob