I get the same error as you. No SSL setup in my case.
==> /var/log/gitlab/gitlab-rails/production.log <==
Started POST "/oauth/authorize" for 127.0.0.1 at 2016-05-05 09:00:20 -0400
Processing by Oauth::AuthorizationsController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"[FILTERED]", "client_id"=>"xxxxxx", "redirect_uri"=>"http://xxxxxx/signup/gitlab/complete", "state"=>"xxxxx=", "response_type"=>"code", "scope"=>"api", "commit"=>"Authorize"}
Redirected to http://xxxxxx/signup/gitlab/complete?code=xxx&state=xxxx%3D
Completed 302 Found in 13ms (ActiveRecord: 2.4ms)
==> /var/log/gitlab/gitlab-workhorse/current <==
2016-05-05_13:00:20.27512 migrationgit.druide @ - - [2016-05-05 09:00:20.252803743 -0400 EDT] "POST /oauth/authorize HTTP/1.1" 302 364 "http://xxxx/oauth/authorize?response_type=code&client_id=xxxx&redirect_uri=http%3A%2F%xxxxx%2Fsignup%2Fgitlab%2Fcomplete&state=xxxxx%3D" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/601.5.17 (KHTML, like Gecko) Version/9.1 Safari/601.5.17" 0.022197
==> /var/log/gitlab/nginx/gitlab_access.log <==
192.168.1.155 - - [05/May/2016:09:00:20 -0400] "POST /oauth/authorize HTTP/1.1" 302 364 "http://xxxx/oauth/authorize?response_type=code&client_id=xxxx&redirect_uri=http%3A%2F%2Fmattermost.druide%2Fsignup%2Fgitlab%2Fcomplete&state=xxxxxx%3D" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/601.5.17 (KHTML, like Gecko) Version/9.1 Safari/601.5.17"
==> /var/log/gitlab/mattermost/mattermost.log <==
[2016/05/05 09:00:20 EDT] [EROR] /signup/gitlab/complete:AuthorizeOAuthUser code=500 rid=xxxxxx uid= ip=192.168.1.155 Erreur lors de la récupération du jeton [details: ]
==> /var/log/gitlab/nginx/gitlab_mattermost_access.log <==
192.168.1.155 - - [05/May/2016:09:00:20 -0400] "GET /signup/gitlab/complete?code=xxxxx&state=xxxxxxx%3D HTTP/1.1" 500 1348 "http://xxxxx/oauth/authorize?response_type=code&client_id=xxxxx&redirect_uri=http%3A%2F%2Fmattermost.druide%2Fsignup%2Fgitlab%2Fcomplete&state=xxxxxx%3D" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/601.5.17 (KHTML, like Gecko) Version/9.1 Safari/601.5.17"
192.168.1.155 - - [05/May/2016:09:00:20 -0400] "GET /static/js/react-with-addons-0.13.3.min.js HTTP/1.1" 404 19 "http://mattermost.druide/signup/gitlab/complete?code=xxxx&state=xxxxx%3D" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/601.5.17 (KHTML, like Gecko) Version/9.1 Safari/601.5.17"
192.168.1.155 - - [05/May/2016:09:00:20 -0400] "GET /static/js/react-bootstrap-0.25.1.min.js HTTP/1.1" 404 19 "http://mattermost.druide/signup/gitlab/complete?code=xxxx&state=xxxx%3D" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/601.5.17 (KHTML, like Gecko) Version/9.1 Safari/601.5.17"
192.168.1.155 - - [05/May/2016:09:00:20 -0400] "GET /static/js/jquery-1.11.1.min.js HTTP/1.1" 404 19 "http://mattermost.druide/signup/gitlab/complete?code=xxxxx&state=xxxx%3D" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/601.5.17 (KHTML, like Gecko) Version/9.1 Safari/601.5.17"