500 error when creating a group with mattermost enabled

Summary

==> /var/log/gitlab/gitlab-rails/production.log <== 
Completed 500 Internal Server Error in 33ms (ActiveRecord: 1.6ms | Elasticsearch: 0.0ms)

==> /var/log/gitlab/gitlab-rails/production_json.log <== 
{"method":"POST","path":"/groups","format":"html","controller":"GroupsController","action":"create","status":500,"error":"Mattermost::NoSessionError: No session could be set up, is Mattermost configured with Single Sign On?","duration":33.66,"view":0.0,"db":1.62,"time":"2019-02-15T17:56:31.948Z","params":[{"key":"utf8","value":"✓"},{"key":"authenticity_token","value":"[FILTERED]"},{"key":"group","value":{"parent_id":"","path":"test500","name":"test500","description":"","visibility_level":"0","create_chat_team":"true"}}],"remote_ip":"10.167.19.235","user_id":237,"username":"asdf"}

==> /var/log/gitlab/gitlab-rails/production.log <==

Mattermost::NoSessionError (No session could be set up, is Mattermost configured with Single Sign On?): 
lib/mattermost/session.rb:89:in `create' 
lib/mattermost/session.rb:39:in `block in with_session' 
lib/mattermost/session.rb:145:in `with_lease' 
lib/mattermost/session.rb:38:in `with_session' 
lib/mattermost/client.rb:12:in `with_session' 
lib/mattermost/client.rb:38:in `session_post' 
lib/mattermost/team.rb:11:in `create' 
app/services/mattermost/create_team_service.rb:9:in `execute' 
app/services/groups/create_service.rb:24:in `execute' 
app/controllers/groups_controller.rb:33:in `create' 
lib/gitlab/i18n.rb:47:in `with_locale' 
lib/gitlab/i18n.rb:53:in `with_user_locale' 
app/controllers/application_controller.rb:333:in `set_locale' 
lib/gitlab/middleware/multipart.rb:95:in `call' 
lib/gitlab/request_profiler/middleware.rb:14:in `call' 
lib/gitlab/middleware/go.rb:17:in `call' 
lib/gitlab/etag_caching/middleware.rb:11:in `call' 
lib/gitlab/middleware/read_only.rb:31:in `call' 
lib/gitlab/request_context.rb:18:in `call' 
lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call'

https://gitlab.zendesk.com/agent/tickets/114445

User is running 10.7. It might be another edge case similar to https://gitlab.com/gitlab-org/gitlab-ce/issues/31835