Please check the topic posted above. For some reason it seems to only affect NT auth people, although it seems it should affect everyone. I checked the code in pop_profile.asp and the data type is correct for Receive Email there.
I'm installing Snitz on an extranet, so I'm using the NT account features. Big problems, like you say here. Whilst I was switching around, trying to circle past the problem, I noticed that the DB (Access) was showing partially completely versions of the unsuccessful registration attempts. Interestingly, some of the text fields were coming back populated with a space character, rather than empty. This rang some bells in my head about Access getting shirty about "non-null" values and "empty" fields. I couldn't talk you through it, but it might be worth checking it out. This may account for why you can't see any bugs in the ASP code - the problem is in the table definition in Access.
Hope this helps.
Thanks for a great product at an outstanding price!
The fix has been posted in the url Nikkol posted in her original topic. More testing would be needed to find out why this bug doesn't affect users that do not have NT Authorization turned on on thier forum.