When renaming a user (from jb1 to jb), I get this error message: There was an error changing your username.
No information is recorded in /logs.
The old username was created because the user used SSO for the first time, and his name was too short. Now, I changed the minimum username length, and wanted to adjust his name.
Changing username minimum after the fact (e.g. setting minimum username length to 4 chars, when you have ten users with 2 char usernames) will cause lots of problems.
I found it: sso overrides username prevents the name change.
I still consider this situation a bug. Enabling this setting should either hide the Change Username button from the administrator panel and disable the users/.../preferences/username page, or still allow username changes for admins (possibly with a warning).
Iād prefer the latter, to allow admins to manually āuntangleā cases where Discourseās change-illegal-names-system has created unintended names.