I just noticed that one of my server instances still ran an older LTS release.
So I did an upgrade to ubuntu 12.04 lts on that OpenVZ container.
Now I cannot install or upgrade any other packages because apt always tries to fix a corrupted installation of initscripts.
Apt always quits with the error:
Setting up initscripts (2.88dsf-13.10ubuntu11) ...
mkdir: cannot create directory `/dev/shm': File exists
dpkg: error processing initscripts (--configure):
subprocess installed post-installation script returned error exit status 1
Do you have any idea how to fix this?