Hi,
We've fixed archive.zentyal.org removing the conflicting packages with the new ones in the Ubuntu 16.04 repos and this should not happen again in fresh installs.
For already broken installations the fix, as pointed out here, is:
wget http://archive.ubuntu.com/ubuntu/pool/main/a/apt/libapt-pkg5.0_1.2.29_amd64.deb
sudo dpkg --install --force-downgrade libapt-pkg5.0_1.2.29_amd64.deb
And after that, as zentyal repo is already fixed, upgrades should work now normally, without need of pining or holding any package.
Thanks for reporting!!
PS: Please also note that Zentyal 6.0, which was released yesterday and is based on Ubuntu 18.04, was not affected at all: The problem in Zentyal 5.1 was caused due to the dependencies introduced for having Samba 4.6 (the Samba version in the official Ubuntu repos was 4.3). In Zentyal 6.0 we are no longer backporting packages, we just depend on the Samba 4.7 packages included in the Ubuntu 18.04 repos (this would be also our policy for all future releases). So, installing Zentyal 6.0 would be the recommended option for fresh installs anyway, as it should not be affected for these kind of issues in the future and just receive updates from Ubuntu normally.