This is the problem!

I looked at the packages for conflict and figured the issue is in gnpug.
Zentyal repository contains gnupg version 2.1.15-1ubuntu6 which breaks
python-apt <= 1.1.0~beta4.

Ok, thank you! Any workaround (something like packege pinning?) to fix this problem?

> And this is a BIG problem ... Can I open a bug ? Where?

Agreed.

Open a bug with Zentyal. They broke the packages from Ubuntu and should
fix it themselves. They have to backport newer version of python-apt.
The one from yakkety (1.1.0~beta5) should be good enough to fix the
problem.

In the bug report note that the ovirt-guest-agent from Ubuntu repository
cannot be installed. It is not only related to the package from the
private repo.

Ok, Thank you!