I went through the process of enabling mattermost in my gitlab install tonight. I configured it using a url, logged in saw it worked. Then decided that url wasn't great. I changed the url (domain) in /etc/gitlab/gitlab.rb and reconfigured gitlab with gitlab-ctl reconfigure. I then tried to login to mattermost using the new url.
It was redirecting me to a gitlab page saying the redirect url was invalid. After checking settings I decided to deauthorize the application in gitlab for my user and under the admin application settings.
Now when I try to login using a gitlab account I get the following error.
Client authentication failed due to unknown client, no client authentication included, or unsupported authentication method.
Anyone have ideas on how to resolve this?