I could fix my WiFi problem.
I've checked the system log (logcat) and saw this line:
WifiHW : Cannot set RW to "/data/misc/wifi/p2p_supplicant.conf": Operation not permitted
So I have backed up p2p_supplicant.conf and deleted it. After a reboot...
Just received a notification from my provider that Huawei updates are now distributed via Hicare only and voilà, there it is. Android 10 for my Mate 20 Pro in Austria :)
I rooted my device now with Magisk.
Here are the correct logs now ;)
Everyone who has the bootloop issue with new v17.0, try the following:
1. Flash Magisk Uninstaller via TWRP
2. Flash new Magisk zip
3. Clear Cache
4. Reboot into the system
5. Again reboot into recovery and flash a kernel (if any)
6. Reboot ...