Fails to mount super partition

[Fabian42]

My device seems to have "super partitions", but TWRP is failing to mount them. The log output after starting TWRP and doing barely anything is this:

Failed to mount '/system_root' (Block device required)
Failed to mount '/vendor' (Block device required)
E:Unable to open /system_root/system/apex
E:Unable to load apex images from /system_root/system/apex
Updating partition details...
Failed to mount '/system_root' (Block device required)
Failed to mount '/system_ext' (Block device required)
Failed to mount '/product' (Block device required)
Failed to mount '/vendor' (Block device required)
Failed to mount '/odm' (Block device required)
...done
Unable to mount storage
Failed to mount '/system_root' (Block device required)
E:Unable to open /system_root
Full SELinux support is present.
Unable to mount /data/media/TWRP/.twrps
Unable to mount /data/media/TWRP/.twrps
Failed to mount '/system_root' (Block device required)

This prevents me from installing an operating system, wiping storage and most other actions. None of the button options can mount these partitions or even purge all partitions and create new ones. I have not tried command-line-based tools, since I don't know how the partitions would have to look if they were not inside a super partition.