Ok, I found the source of the problem, but no solution yet.
I have a non-standard internet connection (DSL/LTE hybrid) which has a lower MTU than usual lines. If I connect to the same RB with the same client config from a different host with an ordinary internet line, the connection is successfully established.
So what I need it to limit the packet size sent by the Mikrotik router. As mentioned I already tried the max-mtu setting which does not seem to have any effect.
Also tried with some mangle rules to tag the connection and pass it to a different bridge, but wasn’t able to get his to run.
I would appreciate if anyone has an idea. Thanks.