After endless tests and reinstallations of ARS and TS, we finally got rid of the TS crash. I can say that mainly the hint from Chichco lead to the solution: start everything as admin. Well, it's here right under your nose and you just can't see it, can you?
Thx for your support everybody.
We are only running into the spam-blocker-feature on our TS server, but from what I read we only have to adjust the options and we should be fine.