Debian – package won’t install correctly using apt-get

Your system is boned in some very curious way. If ldconfig is the cause of a post-installation script returning 135, it's because ldconfig is dying with a SIGFPE -- a floating point exception. That either points to a very curious bug in ldconfig (very unlikely) or some sort of latent hardware error (far more likely).

If you'd like to diagnose this problem further, you can try running ldconfig in a debugger and see what happens, but there isn't going to be a quick fix to this, and my money's still on a hardware fault.

APT is designed to not leave you a broken system. It has no idea that you are running a server, and don't need some package or the other.

You will see behavior like this when trying to remove a package if some other package depends on what you are trying to remove, but can also use another package instead. When you try to remove the package it depends on APT will offer to install the alternative.

You will see this frequently if you try to purge a mail transfer agent (mail server). There are a large number of programs which require a MTA, so when you try to purge exim or postfix the other will be installed.