@Epsilon @erothoff could you please try :
cd /mnt/repo-base && docker-compose exec -u www-data nextcloud php occ config:system:set main_domain
→ should reply “System config value main_domain set to empty string”
Then retry adding an account to a /e/ device (tested with “Murena.io” or “Nextcloud” account type)
Note: for “Murena.io” account type, no need to click on “Use a specific server” if you provide user email address 
Just tested on a freshly installed NC25, without even login on NC web interface first.
If working for you, I’ll open a Gitlab issue.