Login fails with 'Token request failed'; no logs?

After updating my Gitlab installation I have activated Mattermost. I can click the Create new team with GitLab Account button, and Gitlab asks me to authorize accessing the GitLab API.

After that, GitLab sends me back to Mattermost. This is where I run into a problem; MatterMost takes half a minute to respond, and then shows me a HTTP 500 error:

GitLab Mattermost needs your help:

Token request failed

There is no further information, and I can't find any logging for MatterMost on the server. /var/log/gitlab/mattermost/current is empty.

Any idea what I can do to debug this problem?

GitLab 8.1.2