booting hd0a:/bsd: open hd0a:/bsd: Invalid argument

Hi there!

I need help or some hints on how to proceed:
My system a laptop running amd64-current fully encrypted, key-based. I start from BIOS, not UEFI.

Two days ago I updated to the then latest snapshot and apparently s.th. went wrong: At reboot the system stops after

>> OpenBSD/amd64 BOOT 3.34
open(hd0a:/etc/boot.conf): Invalide argument
boot>
cannot open hd0a:/etc/random.seed: Invalid argument
booting hd0a:/bsd: open hd0a:/bsd: Invalid argument
failed(22). will try /bsd
Turning timeout off.
boot>

Has anyone a clue what might have happend and how to solve the issue? I searched the net but didn't find any substantial infos on this. As the error happends with all three USB-keys I have this is unlikely to be cause of the trouble. At present I only have a USB-stick with install62.fs with me (roughly 3~4 weeks old) which only starts if I change from BIOS to UEFI.

As all data from /home is redundantly backed up I could reinstall everything but would prefer to avoid it.

There are dmesgs from this laptop with some older posts. Obviously no current dmesg possible...

Any hint/idea/suggestion welcome!

TIA.

Best,
STEFAN