I was updating a site today to WordPress 3.4, and ran into a strange server error after performing the update that prevented the site from working at all and also prevented the database from updating.
Call to undefined function wp_cache_add_non_persistent_groups()
Took a bit of investigation to figure it out. The issue was that the site was using the ancient wp_cache plugin.
How to fix:
- Remove the line
define('WP_CACHE', true);from wp-config.php
- Delete the
wp-cachefolder from the