[Solved] "The redirect URI included is not valid." after mmost URL change

Hi @Slimmons, you will need to do the following to fix your issue:

1. Login to GitLab as an admin
2. Go to the "Admin area" and click "Applications" in the sidebar
3. Next to the application "GitLab Mattermost" or something of similar name, click Edit
4. Update the URLs you see to match the host of your new mattermost_external_url
5. Click "Submit"

That should fix your issue. Let me know if it doesn't work and I'll help you troubleshoot.