TWRP opening /data/vendor permission denied

[HJarausch]

Hi,
please help!

On my Samsung Galaxy Tab S6 LTE running Android 12,
I have flashed vbmeta_disabled.tar, then I wiped data (factory reset), flashed TWRP-3.7.0_9_15, and rebooted into recovery.
Initially everything looked good, e.g., I could flash Magisk successfully.
But when I try to backup my system I get

Updating partition details ...

Error opening: 'data/vendor' (Permission denied)

and many more such messages.

What am I missing?

Many thanks for some hints or pointers,

Helmut