Humm,
Looking at your paths again,it could be that since it is a relative path and may not always be correct.
try a absolute path.
Like mine would be
/home/mysite/public_html/ubbthreads/jwplayer/jwplayer.js

same goes for the media file path
Or a full url path

you can get a clue from the path setting in the ubb control panel

Blue Man Group
There is no such thing as stupid questions. Just stupid answers