Can you run the UBB.threads Test Script
; it'll list your server max upload values, which cannot be exceeded.
What are your values for attachments in the forum we're working with? How many attachments? How large of files are allowed?
Is your attachments directory chmodded 777? What file extensions do you allow to be uploaded (CP -> Master Settings -> Attachments)?
What is the name of the file (including extension) that you're trying to upload? How large is the file you're trying to upload?