The problems are partially sloved over here.

First off all we added:

&usespaw=1

to all SPAW-related links inside index.php.

That sloved the "invalid session" for many users over here. There were still some users getting that error.

Somehow it was related with the MS IE (6.0.2800 & 6.0.3790) security settings. Changing the internet security settings to "low" removed the error. More strange was after returning the settings back to "medium" the error never showed up again.

Guess ill start to believe in miracles.

There are now only some users left which arent able to save/change TinyD content. Those users have only one thing in common and thats an Squid HTTP Proxy.

Hope it helps.