Solved UFS2 mounted as UFS1 = fs corruption?

Hallo, I've encountered problems in the boot process. The computer was stuck at the bestie splashscreen (not the boot menu, where you chose multiuser or single user mode, the splashscreen that hides the kernel messages). I know that at least i915kms was beeing loaded, as the resolution of the splashscreen got a lot higher in the middle of the process.
Now I decided to mount the partition from an OpenBSD installation to edit loader.conf, so that I could see the kernel messages. After I did that, the FreeBSD bootloader didn't even get past the stage where it says ZFS found one pool, UFS found 2 partitions.
My question is: could mounting the FreeBSD root partition from an OBSD installation have corrupted the FS? I did try to boot from a FreeBSD installation memstick and fsck the UFS partition, but the fsck only complained something about soft updates and exited.
Please help!