I may have a similar problem. The Flash player mode is quite okay but it won’t switch to HTML5 mode (i tried by deactivating flash in firefox, then I refreshed the page). I have a licensed version (5.8), and the jwplayer.js is in the right folder. The plugin page in wordpress seems to see the right version (5.8 licensed).
HTML5 doesn’t work, instead i have a black rectangle with a white play icon. When I click the icon, it sends me to the first file of my playlist (a mp3 file, played by the default Quicktime player)
I work with a Mac and my host is 1&1.
Have you got an idea of what’s going on?
https://www.silkbeats.com/blog