Do you have access to the server logs? Maybe they will show something.
Also, do you have this problem with all images and attachments? If you can narrow down the problem that will help. For example, maybe it only happens with files above a certain size?
Hmm. Well there are legitimate situations where the flash uploader won't work, such as when the forum is behind a directory login. Your problem sounds like something different. I don't know why it's not working, but of course an easy fix is to disable the flash uploader.