Thanks for not leaving me alone.
The phone is "new" to me, thus no data is in danger and I'm ready to do any wipe.
I can boot up the phone to download mode via vol- and power and boot into bootloader from there as well.
Booting into bootloader is NOT the problem.
When booting into recovery from the bootloader, I get the same bootloop as in "normal" boot.
I was on oreo before installing LeeDroid, with OS 3.18.206.1.
That is the reason I can't simply flash an ruu. I don't have a current .206. RUU with the o2___ CID, and older RUUs won't work because of the oreo firmware, as the signature keys are hardcoded in the firmware. Firmware rollback to nougat requires a working twrp.
I did not flash a RUU via fastboot but via zip-on-sd, getting the signature error mentioned before.
I did flash several twrp versions (3.2.1.4, 3.21.3, 3.2.1.2) since the bootloop occured without success.
I also did flash the boot.img I extracted and renamed from the twrp backup.
I.B.