Test case editor fails to load
Problem
Using the Edit action on a Test Case does not result in the Test Case editor loading. The editor fails to load, and displays an error message instead:
Solution
This has been known to be caused by a third party app incorrectly adding JavaScript resources to all Jira web pages:
Xray Test Management for Jira (version 3.3.8 and lower)
Upgrading the above app to a higher version (at least 3.4.0) should solve the problem.