installing apache2-mpm-event on debian fails with unmet plesk-core dependency

[Petros_Mastrantonas]

since we are moving our server setup from mod_php to php5_fpm it also makes sense to not use the apache2 prefork module but move to either the apache2 worker or the apache2 event module.

so when I execute the apt-get install apache2-mpm-event command, apt-get lists the following error msg:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
plesk-core : Depends: psa-phpmyadmin (>= 3.5.2.2) but it is not going to be installed
Depends: psa-phppgadmin (>= 5.1) but it is not going to be installed
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.

so one could say that maybe both packages are not installed. but when i display all my installed packages with:
dpkg --get-selections

both are present:
psa-phpmyadmin install
psa-phppgadmin install

so whats wrong here?