I made a drastic decision.
I removed and reinstalled X-Plane 11.53.
Then I installed my SAAB 340A 1.6.4 and nothing else. EUREKA!!!! It works fine.
Version 1.6.3 is good and 1.6.4 is even better.
EXCELLENT aircraft!
Now I'm going to install the necessary plugins, one by one and check if any of them causes the crash with the SAAB 340A...
If that happens I'll inform you.