Hi.
Here : http://forum.pfsense.org/index.php/topic,6047.0.html
I understand the fact that I shouldn't try to install the miniupnp package in a 1.2 branch release.
But I didn't see any warning whatsoever that I should not perform a firmware upgrade when I have the miniupnp package installed because it will mess things up.
You have to first manually remove the miniupnp package before upgrading the firmware
Anyway, it's solved for me now, but I can imagine that in the future there will be a lot of users that upgrade to 1.2-RC2 or later and will hit the same problem.