I am currently working on the End the War quest and it looks like I finished it; the villagers have redecorated the Town Square and my monarch has approved the new decorations. Neither of the appointed heroes (the monarch and the wizard) have received new assignments, but the quest will not end either. Is there anyone who has come across this problem and knows of a solution?

I'm having the same problem at the moment. From the looks of things after a google search it seems to be when you choose to take over the 2 kingdoms, I haven't heard anyone complain about it not ending after choosing the nicer option yet.
So out of curiosity, which ending did you choose?
It also doesn't seem to matter which heroes you chose for the quest either.
I have an older save that I'm about to drop in to see if I can get it to end with the nicer option, I'm hoping I saved before I made that choice, I'll let you know how it goes.

Hmmm, my furthest away save was still from after he made that choice. I tried playing it to the end again though, still no luck. I also tried removing ALL my mods but again no luck. I'm going to go abandon it I think. I already have plat for this kingdom ambition, and have 6 points still to go. I might try again and choose the nicer option just to see if it ends.
Did you get yours to end yet?

I finally got it to end I was about to abandon the quest but thought I'd give it one last go.
The only things I did differently on this, my 4th attempt to get it to end, were to complete the daily tasks first and get focus up, not that I think either of those things will have made a difference. I also changed the knight back in to the Royal Guard disguise, don't know if that made a difference, or if your wizard will have been given a similar disguise.
I did notice when the monarch went to collect the treasure it loaded the treasure box this time too, which it hasn't before, I've had a loading icon as it loaded in the changes in the town square and the game's been trying to continue in the background so it's had the pop-up of what's in the chest but no actual opening of the chest animation. I'd made sure the camera was facing the direct opposite direction from the town square, as I think the problem may be occuring during that load, but again I don't actually know.(I've never seen a changed area loading in before but twice during this quest I've been where the changes have been made and had this loading icon.)
Those are all the things I did differently this time round when it finally worked. It could just be coincidence though. Try putting the camera where there's nothing going on and you can't see the town square, in the forest or something, so the game has very little to process while that load is going on.
I'd say it's probably glitching during that load or it's something as silly as not still wearing the disguise on your other character.

I chose the nicer option and got stuck.

The quest ended immediately when my 2nd hero (bard) put the guard disguise back on.

Good to know it gets stuck with the nicer option too.

Putting the disguise back on is one of the things I did differently when I got it to end too.

So with 2 of us reporting the same success I'll guess this is the solution to getting it to end.

I think this is the quest people reported as being bugged the most often. It's a shame EA didn't put something about the importance of the disguise in the tooltip for it.

Thank you putting on guard clothes after getting that treasure did the job

Came here to say that having the secondary hero put on guard clothes again worked for me too! (yes in 2022 some people still play TSM )