I was trying to grep on all the data, but it was taking ages to finish.
grep -nr 'yourolddomain%.tld' /etc/yunohost/apps/*/settings.yml was straight forward and really shorter indeed. There is one instance of my old domain name in /etc/yunohost/apps/nextcloud/settings.yml
skipped_regex: myolderaseddomain%.tld/%.well%-known/.*
I removed the line, and then the migration tools_migrations_migrate_forward ended OK.
You were right to point me to settings.yml files!
Thanks a lot!