[imp] Cannot write Autoloader cache to backend.

[Samuel Wolf]

Hi,

I have a lot of this errors in my apache error log:
[Wed Jun 11 23:01:24 2014] [error] [client 192.168.55.104] Cannot  
write Autoloader cache to backend., referer:  
https://192.168.55.45/horde4/imp/dynamic.php?page=mailbox

$conf['cache']['driver'] = 'Null';
$conf['cache']['use_memorycache'] = '';

Whats wrong?

Samuel