I keep getting unexpected game over (all of sudden) in the middle of the fight in front of the gate (first map).
Battle animation plays on and game over is simply displayed (I had 3 characters alive).
This is annoying because it happens all the time. Is that a bug?
Anyone experiencing similar problem?
Thanks.

So you had a man down?

Characters in the game don't die right away, they fall in "agony". I believe that if you have a character in it's agonized state (down but not dead yet) and you let it die (letting the timer run out before finishing the fight, or not performing first-aid) you lose the game.

All four characters have to be alive (agony included) in order to continue. At least that is what I gathered from the in-game tips. I haven't let a character die on it's own like that (my whole team usually just gets slaughtered before I can send someone out to perform first-aid in the tougher fights).

I hope this answers your question. :)

1322 is right,i would only add...if one of your Partymembers goes down a timer should apears above him.
So you have to send him help before the time runs out or win the ongoing fight quickly.

Keep in Mind...that first Aid take some time and that taking damage slows down the progress.