publish blocks entire program

I have a really difficult problem.

I'm using paho-mqtt inside python 3.7 on mosquitto broker at an ubuntu 18.03 system. The program is a bridge between two brokers (which could surely be solved via mosquitto directly, but I did it this way).

One special defined publish command of one client object irregularely hangs the entire program, i.e. starts but doesn't come back. Even timer events are blocked. The only solution is to kill the program from outside. The command itself is obviously ok, since I can send a hundred of publishes without problem. Also try-catch doesn't help, since the pogram hangs. No other publish command of both client object in the program shows this behaviour.

Is there any chance to analyze what happens?

EDIT: I added a threading.Semaphore() on friday - during weekend all seemed to be stable. Migth this be a hint?
EDIT2: Also with semaphore the call to publish blocks / hangs the program.