Found out this error only appears on live-server. On my dev-Server everything works fine. So it should be a server-misconfiguration.

But I checked everything. The module is enabled and the function "apc_fetch" is defined on the live-server....

And a short-test (explained on php.net) also works fine

$bar = 'BAR';
apc_store('foo', $bar);
var_dump(apc_fetch('foo'));

run php -m on your terminal and see if the php apc module is loaded. If yes check the above function exists in that version.

Ok, that's weird. If i test the snippet from above, it works. If I test with "function_exists", the function seems to be defined.

If I run php -m from terminal apc does not exist in the list.

also there are two php.ini files. one for cli and one which loads for apache. http://stackoverflow.com/questions/3871768/2-php-ini-files