OK - update:
I've taken the current snapshot (8th of Spetember) of i386-current and that indeed had another behaviour. It did not crash but neither did it go further beyond the 'entry point at 0x200120' line - - the install simply stops. No panic screen - it just hangs.
The AMD release gets to the 'boot>' prompt. After a short wait it shows a spinning cursor and 1 number (in the other releases it seems like calculating and displays something like '5965416+946088 [61+227984+215947]=0x703e08'). Then it crashes without a panic screen and the system reboots automatically.
Then I've upgraded the BIOS to V1.20 - the latest available. I did have to wipe out OpenBSD and reinstall XP in order to get that done
I've tried out all the different versions of OpenBSD again and I still get the same results. Upgrading the BIOS did not solve it.
Guess it's time to write an email to misc@openbsd ...
Regards,
Marc