MetropleX If you have since visited our live support on [Matrix] and have resolved the problem please let me know so I can mark the issue solved.
Hi, so I managed to flash GOS and boot it. But I don't exactly know how I solved the problem so I'll just tell everything I did:
After I rebooted into the Bootloader I charged the device until I thought the battery would be full. Then I powered off over the volume buttons and the phone lay two or three days untouched in its case.
After that time I started it and tried to flash it over the web installer through the huawai (which afaik was the only device with recognized mine) again. I checked if the huawai had more than 32GB free space - it had. This time, the Download of the image wasn't downloaded anymore. So, I downloaded a new one. That worked really well. Then I started the flashing process. Everything went fine and I didn't have to do anything analog (like bringing the device back into Bootloader after "Writing vendor_boot"). The flashing process finished. Then I unlocked the bootloader and booted into GOS.
Everything went on perfectly then.
But there are still questions that aren't answered:
Why wasn't my phone recognized by other devices than the huawai?
Why did I get the error message? Because the huawai had enough space all the time.