一般的な「userN」ユーザー名

[Johani (Joe)]

Do you have groups with the same names?

This sounds similar to

Username changed after Discourse upgrade

There is likely a group with the name martin. Users and groups cannot share the same name. There was previously a bug which allowed this to happen, but it has now been fixed - that’s why your username has been changed automatically. We should probably update the error message here to make it clear that a username must not match any group names. Added to my list to investigate