Not exactly sure what my issue was.
I had a container that would start. Then I updated Debian inside and afterwards it wouldn't start anymore. But it was not the Debian 13.1 issue mentioned above because I checked and it still said Debian 13.0...
So I tried this and the conversion worked (as per mbr2gpt).
But now, when I start the VM, the console doesn't connect to it anymore. So I don't get to see what is happening in the VM anymore.
(I did switch to OVMF.)
(At first, I tried without...