What helped to me now - hard reboot, after upgrade to systemd formal upstart system, the system can not reboot thru systemd.
After that I remove the requirepass directive in /var/lib/zentyal/conf/redis.conf otherwise I got:
dpkg --configure zentyal-core
Setting up zentyal-core (5.0.14) ...
Reconnecting to redis server (1 try)... at /usr/share/perl5/EBox/Config/Redis.pm line 476.
Reconnecting to redis server (2 try)... at /usr/share/perl5/EBox/Config/Redis.pm line 476.
Reconnecting to redis server (3 try)... at /usr/share/perl5/EBox/Config/Redis.pm line 476.
Reconnecting to redis server (4 try)... at /usr/share/perl5/EBox/Config/Redis.pm line 476.
Redis command 'get version' failed: [get] NOAUTH Authentication required., at /usr/share/perl5/EBox/Config/Redis.pm line 453.
dpkg: error processing package zentyal-core (--configure):
subprocess installed post-installation script returned error exit status 29
Errors were encountered while processing:
zentyal-core
manually started redis with the zentyal config and re configured dpkg... it complained about upstart, but at least moved on:
dpkg --configure zentyal-core
Setting up zentyal-core (5.0.14) ...
Synchronizing state of zentyal.service with SysV init with /lib/systemd/systemd-sysv-install...
Executing /lib/systemd/systemd-sysv-install enable zentyal
initctl: Unable to connect to Upstart: Failed to connect to socket /com/ubuntu/upstart: Connection refused
insserv: warning: script 'lxdm' missing LSB tags and overrides
insserv: Default-Start undefined, assuming empty start runlevel(s) for script `lxdm'
insserv: Default-Stop undefined, assuming empty stop runlevel(s) for script `lxdm'
initctl: Unable to connect to Upstart: Failed to connect to socket /com/ubuntu/upstart: Connection refused
insserv: warning: script 'lxdm' missing LSB tags and overrides
insserv: Default-Start undefined, assuming empty start runlevel(s) for script `lxdm'
insserv: Default-Stop undefined, assuming empty stop runlevel(s) for script `lxdm'
Processing triggers for zentyal-core (5.0.14) ...
* Restarting Zentyal module: webadmin [ OK ]
* Restarting Zentyal module: logs [ OK ]
then you have to manually stop services like bind, samba etc. that were started by systemd at boot to continue with reconfiguration of packages... but still not there to start everything OK.... TBC.
Good luck.