Check in /cache/admin.php that your Moodle system has the 'core coursemodinfo' cache configured to use a file store (not Redis or anything else). This is default, but it's possible you might have changed it. Also check in admin settings / Development / Debugging that debugging is set to 'Developer' level and is set to display in the output. Copy the attached purge-conflict.php file into the root directory of your Moodle testing system. Open 2 different web browsers. Either use 2 different browsers e.g. Firefox, Chrome or else use one 'normal' browser and one copy in private/incognito mode - the point is that you will need two different Moodle sessions at once. In the first browser (Browser A), go to a Moodle page and log in. Get ready to do 'purge all caches' quickly - either from the link at the bottom of the page (if enabled) or from the page in site admin under Development. In the second browser (Browser B), go to /purge-conflict.php - the page should load, displaying a link to 'Go!' Click the link to 'Go!' in browser B. Wait until it starts displaying dots. Quickly purge all caches in browser A. You need to have started, and ideally finished, the purge caches process before browser B finishes displaying dots. Check the purge caches (Browser A) complete without errors (so no 'Directory not empty' errors are displayed). Note: You may still see an error 'failed to open stream: No such file or directory' in browser B. This is probably OK, I got the same error before and after this change.