hmm let me try it and check.
mine worked - no error message .

If a few more will try it and reply, it might help us find out if it is something with your settings or something with the VBbulletin/forum settings.

Do you have a strong pop up blocker?
I don't know if that would cause an error message though.

what happens for me when i click - Manage Attachments
-is a second window opens {pops up}, with the info about sizes and the Upload File from your Computer box