I’ll say this. I tried the workaround by moving from CIFS to NFS.
However now I’m getting errors on any file that have special unicode characters (umlauts).
But it was 76 warnings last backup, and it was 76 warnings the next backup. So at least its consistent.
EDIT: I used FileBot to quickly rename all the offending files to remove the umlauts and re-ran the backup. Ran without errors.
Workaround of moving to NFS works.