FCKeditor + Mediawiki + Cisco WebVPN = editor doesn't load

I've installed FCKeditor extension into Mediawiki 1.12.0 and I'm browsing wiki through Cisco WebVPN connection. If I start to edit a wiki page and click on rich editor link it displays "loading..." and it fires up a ajax post call and it gets a response but FCKeditor doesn't load at all.

I haven't had the time to look what might cause this, but I noticed that WebVPN alters HTML/Javascript in the wikipage and in the FCKeditor frame, which is most likely to cause some problems.

Here is one example how the WebVPN alters the code: http://jarilehtinen.net/debug/fckeditor.txt (code from the iframe)

Any ideas?