Gitlab/Mattermost login error - Bad response from token request

Troubleshooting
2

Hi netgineer,

In this forum post they suggested this fix:

I also experienced this problem when switching my HTTP Gitlab instance over to HTTPS. I had to update my /etc/gitlab.gitlab-secrets.json to use HTTPS for the *_endpoint parameters. I don’t think Mattermost will follow the 302 on the token request if you’re redirecting HTTP to HTTPS. (and rightfully so)

Did you try that?