GitLab as SSO provider for Discourse, registering new user works, but logging in from Discourse redirects to GitLab and the error "The authorization server does not support this response type." is displayed

I'm using Discourse v2.2.0.beta7 +142 and GitLab EE 11.6.3. I installed this plugin: https://gitlab.com/gitlab-org/discourse-omniauth-gitlab, to have GitLab act as an SSO provider for Discourse.

When the enable sso setting is set to TRUE in Discourse, registering new users works, but logging in from Discourse sends me to https://gitlab.example.org/oauth/authorize?sso=REDACTED&sig=REDACTED (I am logged in on GitLab) and the error message "The authorization server does not support this response type." is displayed.

The callback URL is set to https://discourse.example.org/auth/gitlab/callback, trusted is set to "Y", all scopes are enabled. In the Discourse app.yml, I set the application ID, the secret, and the GitLab URL is set to https://gitlab.example.org. If the enable sso setting is set to FALSE, it works just like on forum.gitlab.com.