I disabled all extensions and the FCKeditor worked. I reactivated the extensions step by step and found out that there is a conflict with the semantic mediawiki extension. How can I make sure that both are working properly? Is there a way that the FCKeditor and the semantic mediawiki can use different hooks?
I had a quick look on the sourcecode of the semantic mediawiki (current version). The semantic mediawiki and the FCKeditor extension use one parser hook in common which in itself is no problem.
However, there are more hook calls involved and here we are all in all in trouble as FCKeditor extension bypasses the parser and the semantic mediawiki uses parser hooks for modifying the output text.
Your initial problem was that you did not see any output text when using both extensions. Here I am still lost. Although the extensions cannot work correctly you should see at least something. Provided that this issue could be solved, it might be possible to adapt the FCKeditor extension in such a way that both extension can coexists - at least until the next software update. Hence, sooner or later you will be again in trouble.
Sorry, there is no easy solution to your problem Markus