Hi Chouby,
I'm getting intermittent fatal errors as well:
Fatal error: Cannot use object of type __PHP_Incomplete_Class as array in /home/content/p3pnexwpnas05_data03/05/2268205/html/wp-includes/functions.php on line 3006
These errors happens only for anonymous users. They never happen when I'm logged in as an admin.
I added the define('PLL_CACHE_LANGUAGES', false); setting in wp-config.php but it does not solve the problem.
The host is MediaTemple Premium WordPress, which implements 4 layers of caching (Varnish, Memcached, PHP APC, and storage L2 caching). It seems to be related to caching.
The site is fairly simple with only 3 other plugins activated (Yoast SEO, Yoast Analytics and TinyMCE Advanced).... so no conflict there as far as I know.
Any ideas?