BuddyPress.org

One tip from trying to set this up myself: if your wp-content directory isn’t set to a certain permission level (755 or more likely 777), you’ll have to create the cache directory yourself before the object cache can be written the first time.

To test, load your site after enabling the object cache and see if a directory called cache is created in wp-content. If it isn’t, create it and set permissions for it:

mkdir cache; chmod 777 cache;

Load your site again to make sure that files are getting created inside of the cache directory.