Unfortunately, this has not helped.
I have the apache updated to 2.4.10 and php to 5.5.20.
I have tested it with kernel 3.16.6 / 3.16.7 and 3.18.3.
But at least once a day I got the error in the logs and the server returns from then on only 500 error (in this vhost).
Does anyone have any other ideas?