It worked!
This is the first time since >12 month of trying that a full TimeMachine backup has been sucessfull!
Thanks to @wgstarks and @johnwicker for the good tips!
Solution: I updated to 6.11.2 and added these lines to my /etc/samba/smb-fruit.conf:
# global parameters are defined in /etc/samba/smb.conf
# current per-share Unraid OS defaults
fruit:metadata = stream
vfs objects = catia fruit streams_xattr
fruit:encoding = native
fruit:posix_rename = yes
Edit for a remark:
Even if it did work: The Backup of my ca. 600GB to my Unraid system took approx. 20 hours.