apache enters defunct state on debian jessie (and wheeze)

[Gustav Spellauge]

hello,

after running for several hours apache enters defunct state.

root     14707     1 73 Jan08 ?        2-01:36:44 /usr/sbin/apache2 -k start
www-data 14714 14707  0 Jan08 ?        00:00:01 [apache2] <defunct>
www-data 14715 14707  0 Jan08 ?        00:00:01 [apache2] <defunct>
www-data 14716 14707  0 Jan08 ?        00:00:01 [apache2] <defunct>
www-data 14717 14707  0 Jan08 ?        00:00:01 [apache2] <defunct>
www-data 14740 14707  0 Jan08 ?        00:00:01 [apache2] <defunct>
www-data 14741 14707  0 Jan08 ?        00:00:01 [apache2] <defunct>
www-data 14743 14707  0 Jan08 ?        00:00:01 [apache2] <defunct>
www-data 14744 14707  0 Jan08 ?        00:00:01 [apache2] <defunct>
www-data 14746 14707  0 Jan08 ?        00:00:01 [apache2] <defunct>
www-data 15352 14707  0 Jan08 ?        00:00:00 [apache2] <defunct>

neither apachectl stop
nor systemctl stop apache2.service

will remove the defunct processes

only systemctl stop apache2.service; kill -9 14707; systemctl start 
apache2.service will arrange it so that there is a working 
apache/roundcubemail.

kolab was installled using apt-get

deb http://obs.kolabsys.com/repositories/Kolab:/3.4/Debian_8.0/ ./
deb http://obs.kolabsys.com/repositories/Kolab:/3.4:/Updates/Debian_8.0/ ./

the system is up2date.

what could i do to avoid this unhealthy state to ensure continous 
roundcube operation?

thanks in advance for your advice, gustav