Related issues: "add" button not working, error "We can't find an issue that matches what you are looking for."

Summary

The "Add" button on Related Issues does not do anything.

Steps to reproduce

1. Click on the "+" sign next to "Related issues" under the text at https://gitlab.com/gitlab-org/gitlab-ce/issues/35822
2. Enter "gitlab-ce#35452" in the box
3. Click "Add"
4. Scroll to top, see the error message.

Example Project

https://gitlab.com/gitlab-org/gitlab-ce/issues/35822

What is the current bug behavior?

Error message appears, relation not added

What is the expected correct behavior?

Relation is created between issue and other issue

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com