Thanks for your reply!
Annoyingly, the problem has been there since the very first installation (of course without any plugins). I have already deleted and reinstalled X-Plane. After that I tried the Zibo Mod and updated my video to show the flickering in both aircrafts.
Surprisingly, sometimes when I turn the standby switch from Auto to Off, after a cold & dark start, the flickering disappears. However, this happens very irregularly and only with the default B738. When I start with the engines running, the displays flicker happily throughout the entire flight.
The flickering doesn't seem to follow any rhythm either. Sometimes it is very intense and sometimes there are dropouts for up to 30 seconds at least. Sometimes all displays flicker evenly together and sometimes wildly mixed up. It's just frustrating!
I have already switched from my TV, to which the computer is connected, to a reasonable monitor and tested the HDMI, VGA and DVI in-/outputs. Unfortunately, this had no influence on the flickering.
Except for the flickering, the game runs absolutely flawlessly (with medium settings for my PC).
Maybe it has something to do with the relatively new Ryzen 5 APU, but honestly I am completely confused and helpless!