When you say "update" from Dietpi Buster to Bullseye, do you mean an in-place upgrade or did you create a whole new image on your device and start from scratch?
I ask because your description reads sort of like the circumstance (also in an in-place upgrade) that led to my call for help at
"Unable to install Pi-hole dependency package" during v6 upgrade attempt - #12 by nprampage.
In my case, the root cause was a missing dependency; this message appeared along with the file not found (line 108) message you used. Maybe that thread will help you find what is triggering the error.
Try sudo pihole -r also to see if that gives any additional hints.