Recovering not booting silverblue

I have a silverblue installation that is now in a state where no commit can be booted (using UEFI). Booting either option (without quiet and rhgb options) only shows

EFI stub: UEFI Secure Boot is enabled.

and does not progress further.

I can boot a live cd and inspect the last boot’s logs and it showed, that it rpm-ostree upgraded twice without rebooting in between.

Any Idea on how I can recover the system?