Hangs on Cleaning up Temporary Files

I had a similar issue - the system would throw an error when I tried to publish for HTML5 with Flash fallback. I have mp3 audio on each slide and turned each off one at a time and tried to publish. Turns out it was two out of the 13 audio files that were causing the error. 

On closer inspection, the audio was fine, but the .vtt files created when captions were typed in were somehow problematic. When those were deleted and re-typed, the whole thing worked as planned. 

Not sure if there is a way to create a detection system within the publish or caption writing system, but an alert about problematic caption files would have been useful. When I checked the .vtt files with an online validator, there was no difference between a working one and one that was causing the problem so...not sure what the real problem was, but the .vtt files were the cause. 

Hope that helps someone.