Log in as admin Navigate to Plugins > Caching > Configuration in site administration Under Summary of cache lock instances select Add a new lock instance: File locking Name: Mylock Under Installed cache stores press Add instance in the APC user cache (APCu) row Store name: Myinstance Locking: Mylock Under Known cache definitions press Edit mappings in the Activity completion status row Primary store: Myinstance Press Delete store in the Configured store instances table for Myinstance Confirm you see the following error: You cannot delete this store because it has mappings. Please delete all mappings before deleting the store Manually navigate to the following URL: <WWWROOT>/cache/admin.php?lock=Mylock&action=deletelock Confirm you see the following error: You cannot delete this lock instance because it is being used by one or more stores.