Sorry, I saw 5.x in the requirements and assumed it was referring to PHP It sounds as though the script(s) aren't reading POST data correctly. Depending on your programming/scripting experience you could dig around in the code to find and fix the issue. Failing that try another discussion forum as such as phpBB or punBB.
I have trouble believing it's a code issue; IPB is a high-end, commerical message board similar to vB. Thanks for the help, I'll just contact invision support; i was just hoping i could get a quick answer here.