I have made some further experiments:
the bug is still occuring with the latest updates.
I have traced the problem down to the main apache functions. It is not a mod_ssl issue!
What is actually happening, is that due to "a2enmod ssl", the server have to listen on two ports: 80 and 443. This activates AcceptMutex to synchronize running processes. And here some problem occures under Ubuntu 12.04
So an alternative way to reproduce is:
$ a2dismod ssl
$ echo "Listen 81" > /etc/apache2/ports.conf
$ service apache2 restart
$ ab -c 1000 -n 5000 http://localhost/
Changing the "AcceptMutex" config option causes different results. For example with:
i get all 5000 requests done without error. Then trying to run ab again causes apache to stop accepting requests.