Sony Bravia KDL-60W605B "File not found." message when trying to play MP4 format vids

[armchairdriver]

TV: Sony Bravia KDL-60W605B
Profile: Sony Bravia KDL (2014 models)

I have an issue with trying to play MP4 (and some MKV) format videos on the TV through Mezzmo (v4.1.3.0). It seems to play other formats fine, but some of the MP4 and MKV just won't play at all with "File not found." appearing on the TV. Trying the "Play To" option in Mezzmo comes up with Error 501 (typically "getvolume failed" or "invaliddatetime")

*** FOUND POSSIBLE CAUSE - I had the "do not stream titles that are unrated" box ticked and although the files that wouldn't play back before had a parental control (PG, 15, 18 etc) rating set by the Maintain Files option (or set manually by me), some of the files don't appear to see this rating because they wouldn't play back. Unticking this box made them play again.