A Useless Error Message

While doing basic edits on a movie using iMovie 09, I got this useless error, “Alert, Error Occurred” while trying to upload to YouTube. That’s it, no reason or error codes. Searching for these terms though, I did find the culprit in this Apple Discussion forum.

Re-enter your YouTube account credentials, possibly using your YouTube-associated email address, and things should work. But how would you guess that from that useless error?!