Installing and upgrading help

I think this is due to a bug in Moodle so I've added a Tracker issue: MDL-64719.

If you're comfortable editing your site's source code you can make the following change to the function user_create_user() in user/lib.php, looking for the following lines (should be lines 129 to 131:

    // Purge the associated caches for the current user only.
    $presignupcache = \cache::make('core', 'presignup');
    $presignupcache->purge_current_user();

and changing the last line so this becomes:

    // Purge the associated caches for the current user only.
    $presignupcache = \cache::make('core', 'presignup');

    if (get_class($presignupcache) !== 'cache_disabled') {
         $presignupcache->purge_current_user();
    }

I tested this and I was able to install Joomdle after this change.