With logs set to debug I have the following:
[root@gitlab mattermost]# tail /var/log/gitlab/mattermost/mattermost.log
[2015/12/09 12:52:17 UTC] [DEBG] Parsing server templates at /opt/gitlab/embedded/service/mattermost/api/templates/
[2015/12/09 12:52:17 UTC] [DEBG] Initializing web routes
[2015/12/09 12:52:17 UTC] [DEBG] Using static directory at /opt/gitlab/embedded/service/mattermost/web/static/
[2015/12/09 12:52:17 UTC] [DEBG] Parsing templates at /opt/gitlab/embedded/service/mattermost/web/templates/
[2015/12/09 12:52:17 UTC] [INFO] Starting Server...
[2015/12/09 12:52:17 UTC] [INFO] Server is listening on :8065
[2015/12/09 12:52:17 UTC] [INFO] RateLimiter is enabled
[2015/12/09 12:52:21 UTC] [DEBG] /
[2015/12/09 12:52:22 UTC] [DEBG] /api/v1/admin/log_client
[2015/12/09 12:52:22 UTC] [EROR] /api/v1/admin/log_client:client code=500 rid=w7wpomfuhfdnmbarmrgqokbafe uid= ip=10.20.1.1 msg: Uncaught TypeError: Cannot read property 'page' of null row: 15 col: 21325 stack: TypeError: Cannot read property 'page' of null url: https://chat.mydomain.vm/static/js/bundle-4063.min.js [details: ]