This error occurs when a user creates an account with email/password and attempts to log in to GitLab Mattermost using the GitLab SSO option.
In GitLab 8.0 omnibus, this should no longer be an issue since the default install will only permit GitLab Mattermost account creation via GitLab SSO (remove confusion by remove email-based account creation in default setup).
Also, Mattermost will be updating the error message for v0.8.0 to make the issue more clear.
Hi @folpindo, this isn’t an issue with Mattermost, the fix was needed in GitLab omnibus configuration and the correction has already been made. If you run the GitLab 8.0 omnibus, which ships on the 22nd, you shouldn’t have this issue.
Mattermost is making the error message more clear in v0.8.0