Solved following a hint by @marc in thread Yunohost 2FAuth Failed, by manually inserting a user in the database
INSERT INTO `users` (
`id`
, `oauth_id`
, `oauth_provider`
, `name`
, `email`
, `email_verified_at`
, `password`
, `is_admin`
, `remember_token`
, `created_at`
, `updated_at`
, `last_seen_at`
, `preferences`)
VALUES (
NULL
, '1'
, '1'
, 'Boudewijn'
, 'wbk@mydomain.tld'
, NULL
, '$2y$10$r45mZlCgEbQ69DMGTpT1Q.xCMjyp5P53kgtkVL/ThOMV7v8KngvXm' --> use a bcrypt hash generator
, '1'
, NULL
, NULL
, '2025-07-12 20:54:44'
, '2025-07-12 20:54:44'
, NULL)
The installation log actually states the creation of the user table and marking of users as admin, as well as checking that the 2fauth user is a Yunohost-user, but even after multiple re-installations (also with more than one user as a test case), the problem persisted on my server. Running the above query solved the problem.