An update over this software update:
I spoke with the seller of my Fairphone 3+ about the issues I’m experiencing. They offered a return for a new Fairphone.
So I did a stock ROM flash, version 8901.3.A.0054.20200929. Curious as I am, I tried a WhatsApp video call and everything works fine. Also the issue of poor video quality during a videocall, which I mentioned in the LineageOS 17.1 thread -here- disappeared.
So I’m going to flash the old version again -yay) before this software-update and keep it until a proper solution arrives.