I've never seen this (and I've done a LOT of installs
The rpm db may indeed be bad, though if it allowed you to install a different RPM, it's _probably_ ok.
Try this:
rpm -e --noscripts --allmatches zimbra-core
If that works, install by hand:
rpm -i -vv zimbra-core-whatever.rpm
Try to see where that hangs, if it does.
If it does hang, repeat the above rpm -e, then reinstall without the scripts:
rpm -i -vv --noscripts zimbra-core-whatever.rpm
If that works, then the problem is in the pre or postinstall scripts of the rpm, and we can track that down.