I have now also tested with vdisk using cache='none' again. This time without any errors. Yay!
I think I can finally conclude that my problems were all hardware related and that new RAM modules have fixed the problem - at least for now. It is still quite strange to me that all of my four RAM modules apparently failed. That suggests an external cause. But I have no clue what that cause might be. I guess I just have to wait and see if it happens again.
pervel's post in Lots of BTRFS errors was marked as the answer