Enable performance info if necessary (in admin menu, search for perfdebug admin setting and enable). View a course page. In another tab, open the Purge caches page (admin/purgecaches.php - admin settings / development / purge caches on the menu). Under Purge selected caches heading, tick All MUC caches and press Purge selected caches . Back in the first tab, reload the course page. Look at the performance information at the bottom of the page. EXPECTED: You should see a table similar to the one shown in the screenshot for this issue, listing at least one lock that was obtained, the time it took to obtain, a tick indicating that it was obtained successfully, and the time it was held open for. (Unless you have a complicated test site, most likely the times will be 0.0 or 0.1.) Now edit your Moodle config.php and add the following line before the require_once at the end: define('MDL_PERFTOLOG', true); Purge the MUC cache again, and reload the course page again. Look near the end of your web server error logs. EXPECTED: You should see a line relating to the course page (look for the course/view.php), which should contain the same lock information in text format at the end of the line, something like this: Locks (waited/obtained/held): core_coursecattree_cache (0.0/y/0.0) build_course_cache_1 (0.0/y/0.1) build_course_cache_2 (0.0/y/0.2). When you've finished testing, remember to remove or comment out the added line from your config.php.