So, I have now tried:
- Linux Mint (kernel 6.8)
- Sysrescuecd
- Windows 11 (through VirtualBox)
…but nothing helps
The device works otherwise well.
UPDATE: I managed to unlock the bootloader by adb reboot bootloader and/or recovery mode (power+volume down)->fastboot->bootloader and alternating USB cables. I had to do something similar in the middle of flashing. Not fun, something is very fiddly with my device 