You may encounter the following situation:
The Quick Preview ([Return] key) and flash render does not open the preview window after being performed.
This is caused by some QuickTime version 7.1.3 default preferences disabling the playback of Flash movies.
You can remedy this by manually re-enabling the playback of flash tracks in the Preferences menu option.
To re-enable it, do the following:
If you have upgraded to QuickTime 7.3.1 it no longer supports Flash tracks. This makes the quick preview feature unusable in Toon Boom Studio. The Preview Scene and Preview Movie features will still work, so you could use these meantime.
If this solution is unsatisfactory, then roll-back your version of QuickTime to a previous one (between 7.1.3 and 7.3.0) on Windows or roll back your OS to a time before you installed the newer version of QuickTime.
Mac OS X (all)