Hi jess,
thanks for your help. Obviously the error was due to missing wheezy repositories. The installation continues now, but fails again during kaltura-nginx install... Here's what's happening in the console 
The following NEW packages will be installed:
kaltura-nginx
0 packages upgraded, 1 newly installed, 0 to remove and 7 not upgraded.
Need to get 558 kB of archives. After unpacking 1,333 kB will be used.
Get: 1 http://installrepo.kaltura.org/repo/apt/debian/ kajam/main kaltura-nginx amd64 1.8.0-8+b7 [558 kB]
Fetched 558 kB in 1s (504 kB/s)
Preconfiguring packages ...
Selecting previously unselected package kaltura-nginx.
(Reading database ... 109589 files and directories currently installed.)
Preparing to unpack .../kaltura-nginx_1.8.0-8+b7_amd64.deb ...
Unpacking kaltura-nginx (1.8.0-8+b7) ...
Processing triggers for systemd (215-17+deb8u3) ...
Setting up kaltura-nginx (1.8.0-8+b7) ...
Job for kaltura-nginx.service failed. See 'systemctl status kaltura-nginx.service' and 'journalctl -xn' for details.
kaltura-nginx.postinst FAILED with: 41233 on line 68
Archving logs to /opt/kaltura/log/log_01_03_16_18_54.tar.gz...
dpkg: error processing package kaltura-nginx (--configure):
subprocess installed post-installation script returned error exit status 11
Processing triggers for libc-bin (2.19-18+deb8u3) ...
Errors were encountered while processing:
kaltura-nginx
E: Sub-process /usr/bin/dpkg returned an error code (1)
Failed to perform requested operation on package. Trying to recover:
Setting up kaltura-nginx (1.8.0-8+b7) ...
Job for kaltura-nginx.service failed. See 'systemctl status kaltura-nginx.service' and 'journalctl -xn' for details.
kaltura-nginx.postinst FAILED with: 41296 on line 68
Archving logs to /opt/kaltura/log/log_01_03_16_18_54.tar.gz...
dpkg: error processing package kaltura-nginx (--configure):
subprocess installed post-installation script returned error exit status 11
Processing triggers for libc-bin (2.19-18+deb8u3) ...
Errors were encountered while processing:
kaltura-nginx
root@myserver:~# journalctl -xn
-- Logs begin at Tue 2016-03-01 18:06:19 CET, end at Tue 2016-03-01 18:54:25 CET. --
Mar 01 18:32:09 myserver /etc/mysql/debian-start[30547]: This installation of MySQL is already upgraded to 5.5.47, use --force if you still need to run mysql_upgrade
Mar 01 18:32:09 myserver /etc/mysql/debian-start[30582]: Checking for insecure root accounts.
Mar 01 18:32:09 myserver /etc/mysql/debian-start[30587]: Triggering myisam-recover for all MyISAM tables
Mar 01 18:35:22 myserver monit[31511]: Stopping daemon monitor: monit.
Mar 01 18:35:22 myserver monit[31535]: Starting daemon monitor: monit.
Mar 01 18:39:01 myserver CRON[35297]: pam_unix(cron:session): session opened for user root by (uid=0)
Mar 01 18:39:01 myserver CRON[35298]: (root) CMD ( [ -x /usr/lib/php5/sessionclean ] && /usr/lib/php5/sessionclean)
Mar 01 18:39:03 myserver CRON[35297]: pam_unix(cron:session): session closed for user root
Mar 01 18:54:20 myserver systemd[1]: Unit kaltura-nginx.service cannot be reloaded because it is inactive.
Mar 01 18:54:25 myserver systemd[1]: Unit kaltura-nginx.service cannot be reloaded because it is inactive.
root@myserver:~#