Questions: same movie, same PC, no changes meanwhile such as perhaps other filters/codecs installed? E.g. if you now re-install 2.15 the movie is indeed still of a better quality?
Check which filters BS.Player uses for version 2.15 respectively 2.21:
(rightclick >) Options > Filters > Advanced
Any difference between the two versions?
Same for settings at [Video rendering]-page:
(rightclick >) Options > Preferences > Video > [Video Rendering] ?
(What rendering mode is used?)
P.S.:
Assuming you have a legally obtained PRO-version: did you know you are entitled to get direct e-mail support by BS.Player-team?
You may contact them through the contact form:
http://www.bsplayer.com/en/contact_us/ (or alternatively you can send them an e-mail at
customer.support@bsplayer.com )