The mariadb_upgrade_wizard process fails upgrading to any 10.6.x if "old_mode=" is configured on my.ini.
Removing old_mode the upgrade works fine and can be set again after the update has finished.
The problem is when upgrading from 10.5 to 10.6, the wizard does not ends and the service is still pointing to 10.5.
I attach the error message and the server's log.
Hope this will help.