Set `$CFG->enable_read_only_sessions = true;` in config.php. You may get an exception about cache mappings. If so, disable the above setting and follow these steps: Go /cache/admin.php Under "Installed cache stores", find "File cache" and click "Add instance Set store name to "Session cache", Cache path to "/tmp/session" and click "Auto create directory". Save changes At the bottom of the page, under "Stores used when no mapping is present", click "Edit mappings" Set "Session" to "Session cache". Log out and close your browser (or open a new browser/private window). Add `$CFG->enable_read_only_sessions = true;` in config.php and load the site again. You should no longer see an exception message. Set `$CFG->debugsessionlock = 0.1;` in config.php. Open a terminal and tail your server's error log. For example: tail -f /var/log/apache2/error .log Go to /admin/settings.php?section=manageantiviruses on your Moodle site. This page includes an AJAX request to the core_check_get_result_admintree external function, which has `readonlysession` set to true. Check the file that PHP error logs are written to. Confirm that it does not contain a message like a message like " Debugging: Script /lib/ajax/service.php?sesskey=xxx&info=core_check_get_result_admintree locked the session for 0.458 seconds, it should close the session using \core\session\manager::write_close()". This confirms that debugging is not triggered for read-only sessions. Visit /admin/tool/generator/maketestcourse.php Confirm that the page contains a message like "Script /admin/tool/generator/maketestcourse.php locked the session for 0.604 seconds, it should close the session using \core\session\manager::write_close()." This confirms that debugging is still working for locked sessions.