ANSWER:
Yes, that is it. Now I have corrected my yunohost instance and it shows; Powered by YunoHost 11.0.10.2 (stable).
The actual CORRECTION:
Find file: /etc/yunohost/migrations.yaml
Remove completely the line: 0021_migrate_to_bullseye: skipped
After that you are able to run the migration again in yunohost webadmin.