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.
Anyway, updating PVE (including pvecontainer from 6.0.9 to 6.0.11) helped and the container is now starting again. No host reboot necessary.
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.
Anyway, updating PVE (including pvecontainer from 6.0.9 to 6.0.11) helped and the container is now starting again. No host reboot necessary.