I've created a bugreport --> https://bugzilla.proxmox.com/show_bug.cgi?id=6733
Unfortunately I did not have time yet to test the commands for Fabian. Maybe you can contribute to this bug report too :)
Tried your suggestion and that worked (I'm using pve-nosub)....
However, how to go on from here? Stick to the older grub2 and not update to the newer ones until further notice?
I'm running Proxmox on an older Dell R530 (Proxmox 5 at setup). I tried updating BIOS (as AI suggested) but this did not work out. All the 6.x Kernels weren't bootable. This problem did not occur on my Intel Nuc13 Cluster (Proxmox 8 at setup)...