Hello Florian,
ok, that’s definitely a good hint - I will check the new version and the corresponding config options.
And thanks for pointing to the event.log - I totally had forgotten about that logfile. 
After activating DEBUG level, I found the corresponding number of messages like this:
2020-02-12 16:34:54,411 DEBUG - Outgoing publish message was dropped. Receiving client: {client_id}, topic: {topic}, qos: 0, reason: The tcp socket was not writable.
It seems our client is not consuming the messages fast enough (a hint I already got from your colleague Michael in this topic). I definitely need to check this.
What will happen, when we switch back to shared subscriptions? Will the broker then skip a blocked client in the subscriber-group and distribute the messages among the other subscribers from the same group?
Finally: No, I didn’t explicitly specify -Xms or -Xmx parameters.
Greetings!
Reiner