Fatal error: Call to undefined function cache_get()
For me, the cause was different: some errors are not caught by Drupal's error handler.

I used a new DateTime() in my settings.php (probably very rare)
It generated an error because the date.timezone setting in my CLI php.ini wasn't set.

To find the real error (this is the interesting part of the answer for all of you, I think) :

Edit your module.inc, line 723.

Add these lines before the cache_get call.

if (!function_exists('cache_get')){ 
    debug_print_backtrace(); 
}
$implementations = cache_get('module_implements', 'cache_bootstrap');


It might give you lots of information about the real source of the issue!