Hi, thank you all for your replies.
I got the problem and all of you are right! Unfortunately it leads to the same issue mentioned here: Issues with Mail responding.
All messages will be moved and filtered correctly if sended from delta but NOT if replied by a standard email application. In this case a Chat-Tag in the Mail-header is missing, hence a server side rule with Chat-Version (or similar) will not work.
Thus what exactly is the tag in the mail header by which delta recognize it as chat-message? And how has this changed in the latest version (with an older version it works perfectly)?
(Main issue for this thread is solved)