I tried the MacOSX install disk, which does have a disk check and repair tool. It found some errors on the mac partition, but was unable to repair it.
After this I decided to get rid of MacOS and booted MorphOS, deleted the MacOS partition and created a sfs work partition instead.

Now it wont boot MorphOS anymore, so it looks like I messed up something. It's no disaster, since I was thinking about starting from scratch anyway when updating from 3.11 to 3.12.
But I do have a lot of stuff on that disk I want to preserve, so now I have to either manage to reconstruct a working disk layout manually without loosing my data, or copy everything to a usb-stick and let the MorphOS installer do all the setup.