Savegame: http://dl.dropbox.com/u/9164057/FE/lala.EleSav

In this savegame the player immediatly faces tactical combat. During this combat, when inspecting an enemy (or presumably also a friendly unit), a diplomacy screen will pop up. Ever after this combat will not progress correctly. After selecting actions, the units will stand around without doing anything until you inspect a unit again.

This keeps on happening until combat is over.

I assume that outside actions (like first discovery of another nation) intrude their thread into the combat threads - so in order to replicate this, you might want to make a map close to enemies and start combat every turn until you get the same issue.