Client authentication failed after installing Omnibus GitLab 8.9

Hi,

I have successfully installed GitLab 8.9 using the Omnibus package.

After I managed to resolve the 502 error (the permissions on /var/log/gitlab/mattermost were wrong) I'm getting Client authentication failed due to unknown client, no client authentication included, or unsupported authentication method. when trying to sign in using GitLab.

Mattermost relevant lines from gitlab.rb:

mattermost_external_url 'https://m.mydomain.com'
mattermost['enable'] = true 
mattermost['username'] = 'mattermost'
mattermost['group'] = 'mattermost'
mattermost['home'] = '/var/opt/gitlab/mattermost'
mattermost['database_name'] = 'mattermost_production'

mattermost['service_use_ssl'] = true 
mattermost['service_enable_incoming_webhooks'] = true
mattermost['team_site_name'] = "Mattermost @ MyDomain"
mattermost['gitlab_enable'] = true
mattermost['gitlab_secret'] = "sekrit"
mattermost['gitlab_id'] = "even more sekrit"
mattermost['gitlab_scope'] = ""
mattermost['gitlab_auth_endpoint'] = "https://git.mydomain.com/oauth/authorize"
mattermost['gitlab_token_endpoint'] = "https://git.mydomain.com/oauth/token"
mattermost['gitlab_user_api_endpoint'] = "https://git.mydomain.com/api/v3/user"

mattermost_nginx['enable'] = true
mattermost_nginx['redirect_http_to_https'] = true
mattermost_nginx['ssl_certificate'] = "/etc/letsencrypt/live/m.mydomain.com/fullchain.pem"
mattermost_nginx['ssl_certificate_key'] = "/etc/letsencrypt/live/m.mydomain.com/privkey.pem"

What am I doing wrong?