This mysterious error through me for a loop today. I had a flash application that needed a progress bar to show loading status. There may be other solutions out there, but we made our own. It didn’t take long, just created an index.fla that then loaded the big swf and then displayed the progress bar. No big deal. Everything seemed to work just fine, but there was as minor glitch the scroll bar wasn’t showing up. Everything else was working, but the UIScrollbar was generating the above error.

After some experimentation, I discovered that the UIScrollbar has to be in the Library of the index.fla. Don’t know why this is, the UIScrollbar was included in the swf that was loaded, but hey, this is about making things work not figuring out exactly why it doesn’t work.