Hmm, it did work for me the one time I got that quest.  I had a dead rat ("for proof") in my inventory until I got back to the inn.  But perhaps it's an intermittent problem like the damsel in distress seems to be, where sometimes you get the reward and sometimes you don't.

In my game when I attempted to land on the Inn's square to turn the rat in my dude would overshoot it like he was on a water slide going into a square next to the inn. So I was unable to complete it since I couldn't actually land on the Inn anymore.

Another quest I couldn't complete was rescuing some noble's son. My troops had the unit in their stack and when they brought him back to my town the unit just sat in my town and there was no completion.

I have done it two or three times with 0.3 and has worked fine. (The Damsel and Knight quests have worked fine too)

I've been able to complete a couple of those quests by using the raise land spell to allow me to get to where I needed to go.  I think both of them were escorting someone to an estate for the reward.  Of course first I had to travel down to the southwest where the earth shard is and defeat whoever was there.

OK, scratch this idea.  I just failed the rats quest again (i.e. I carry the dead rat back to the inn and nothing happens), but it didn't involve loading a saved game in between getting the quest and completing it.  I had loaded a saved game, but it was before I got the quest.