HTTP 500 error when loading editor CSS in Moodle 3.4

Theme Boost Make sure theme designer mode is off (make sure Moodle is using caches) Set TinyMCE as the default editor Site admin > plugins > text editors > manage editors Click the “up arrow” to move TinyMCE to the top of the list Purge the Moodle caches Open the dev tools in your browser and view the networks tab and filter for CSS Create a new course CONFIRM that the “editor” stylesheet returns a HTTP 200 status (note: there will be no data in the response, this is a Boost specific issue - see MDL-60588 ) View the localcache on disk <moodledatadirectory>/localcache/theme/<revision>/boost/css/ CONFIRM there is an editor CSS file that contains a revision in the name editor_<revision>.css Theme Clean Make sure theme designer mode is off (make sure Moodle is using caches) Set TinyMCE as the default editor Site admin > plugins > text editors > manage editors Click the “up arrow” to move TinyMCE to the top of the list Purge the Moodle caches Open the dev tools in your browser and view the networks tab and filter for CSS Create a new course CONFIRM that the “editor” stylesheet returns a HTTP 200 status and that the response body contains CSS View the localcache on disk <moodledatadirectory>/localcache/theme/<revision>/clean/css/ CONFIRM there is an editor CSS file that contains a revision in the name editor_<revision>.css In the “Course Summary” section start, add some elements (e.g. paragraph, heading 2) and confirm that the styling looks the same as those elements on the rest of the page (e.g. the heading 2 should be the same size as the other h2 on the page)