If you change background color, does it really change?
Do BSPlayer's shortcut-keys for subtitles work?
(E.g. Shift+Up resp. Shift+Down to move subtitles up/down
or Shift or Ctrl in combination with left/right arrow-key to timeshift/sync subtitles with movie)
What is selected under
(rightclick >) Options > Preferences > Video > Videorendering?
(Is "Draw subtitles to Overlay surface" selected under Preferences > Subtitles?)
(Perhaps also read
this thread)