Problem:
JW player didn't play any video in Chrome / Firefox (Windows). All video's resulted in a black box without controls
Playback in Internet explorer worked fine.
Setup:
WordPress Multisite (3.2.1)
JW Player for WordPress – Flash & HTML5 Video Player 1.4.3
Solution:
JW player was only activated on site level. (had been working fine before)
Problem was solved after Network activating JW Plugin on entire WordPress setup.