Enable readonly sessions debugging and session lock debugging in config.php: $CFG->enable_read_only_sessions_debug = true; $CFG->debugsessionlock = 100; Tail the error logs in a new terminal window: tail -f /var/log/apache2/error.log Visit the dashboard and some other random pages to fill the recent session lock list Copy attached php file recentsessionlocks.php to your site root and visit it in your browser Confirm there are not any errors in the logs like this: Script [your site root]/recentsessionlocks.php defined READ_ONLY_SESSION but the following SESSION attributes were changed: $SESSION->cachestore_session $SESSION->recentsessionlocks $SESSION->foo NB: The error about SESSION attributes being modified will still be present, but what needs to be verified is that $SESSION->recentsessionlocks is not in the list of modified attributes.