Support

FOF 3.0 is installed with the Akeeba Backup or Admin Tools package. If you get an error about it missing which "suddenly" goes away without doing anything the culprit is your server, namely PHP op-code caching. When you ask PHP to do something it first reads and compiles the .php file into an intermediate form called "op-code", then executes the op-code. An op-code cache essentially makes PHP "remember" the op-code format of a .php file so it doesn't have to do the repeated and computationally expensive first part (read and compile the file) all the time. This makes the site snappier. The downside is that whenever you install or update anything on your site you must clean the op-code cache.

Unfortunately, even though hosts let you enable op-code caching, or do it for you without asking you, they invariably fail to tell you the all-important fact of needing to clear the op-code cache. I suggest asking your host how to do that and then simply remember to clear the op-code cache every time you install an extension OR whenever you update either Joomla itself or one of its extensions (components, modules, plugins, templates, ...). This will preserve your sanity. Trust me, I know...

You're welcome :)

For what it's worth, the problem is not unique to Joomla. Op-code caches affect all PHP software, either mass distributed (like Joomla, WordPress, Drupal, ...) or bespoke. What usually happens is that the folks making bespoke software, as well as Drupal site administrators, are aware of the op-code caching and do tricks to reset the cache and "warm it up" at the time of new code deployment. The rest 30% of the web is blissfully unaware for the most part ;)