Well, I’ve scrutinized the problem once more and found, the CKEditor for WordPress caused it. This plugin has not been refreshed since ten months; the developer is looking for a new maintainer. In a nutshell, the newest verions of the FV WordPress Flowplayer doesn’t work together with the old versions of CKeditor. Switch out the CKeditor, turn back to TinyMCE – and everything should work.