While (ab)using TinyMCE all of the sudden it stopped working and there was no error/notice in the JS errors Console. I've tried several things without success and after 20 minutes I've thinket to start "tamper data" in firefox to check what was wrong. The problem was quite simple ... a plugin was completely missing (404 page). If TinyMCE doesn't start check the errors console, missing files, missing div/textarea with relative ID.