"Could not connect to the CI server" on a project with pipelines disabled
After upgrading from GitLab Omnibus 8.5.8 to 8.5.10, and then immediately upgrading again to 8.6.4, the error message "Could not connect to the CI server. Please check your settings and try again." is shown on every merge request. This happens regardless of whether the merge request was created before or after the upgrade.
Builds were enabled for this project (per the default settings), but no runners were ever set up. I have now disabled builds for this project and the error still appears. All the CI settings are at their default values.
Please let me know if I can provide any information to help track down this issue.
May be related to #14589 (closed), but unlike that issue this happens every time, and no runners are configured.