'Cannot connect to the CI server' in merge requests

Summary

GitLab displays Cannot connect to the CI server. Please check your settings and try again. in new merge request build info header even though

1. GitLab CI is properly set up and reports success on commit builds
2. when pushing new commits to existing merge request, merge request build info becomes green with expected message CI build passed for 830a9895.

How can I debug this further? Did not see errors in /var/log/gitlab/gitlab-rails/production.log, should I look somewhere else?

Versions

• gitlab_7.9.2-omnibus-1_amd64.deb
• gitlab-runner_5.2.0~omnibus.1-1_amd64.deb

Steps to reproduce

1. Setup GitLab CI according to official instructions
2. Install and setup GitLab runner according to official instructions
3. Login to GitLab CI by pressing Login with GitLab, authorize access from GitLab
4. Add any project from GitLab to GitLab CI by pressing Add project to CI
5. Verify that the runner and default job are active in GitLab CI in /projects/1/runners and /projects/1/jobs
6. Verify that GitLab is able to connect to GitLab CI in /project-name/core/services/gitlab_ci/edit by pressing Test settings
7. Push to master, verify that the commit is visible in GitLab CI in /projects/1 and the build is green
8. Create a merge request in GitLab

Expected behavior

Build is triggered for the newly created merge request and build results reported in the merge request header.

Observed behavior

GitLab displays Cannot connect to the CI server. Please check your settings and try again. in merge request header.

Comments

Had to manually install Thawte SSL certificates our sites use, didn't work without these.