I'm having the same problem. I used the pre-made stall which is enclosed on the pre-made rooms. I placed two down. Both horses will go into one stall, the other stall neither of them will go in and I get the same glitch as the OP posted. The small farm animals will go into the other stall which no horses will go into. They're both identical, so it makes the response of the stall not being big enough redundant. Not sure what to do. I decided to delete one of the stalls and extend the other stall and placed another hay bed down and they both will sleep in there now, so I've managed to get around the problem but there's clearly bugs.
@ShadowBramble Hi! I encountered this issue too! I wasn't sure which of the pre-mades you were having trouble with so I tried both the Steed Sanctuary and the Simply Stable Personal Paddock. The horses were able to enter both of the Sanctuary stalls without a problem. However, I did have an issue with the Personal Paddock stalls. Both of them could enter the first one I placed but when I tried to call one of the horses to the second stall next to it, the horse got the routing failure.
First, I checked if the gates were locked. They were so I unlocked them but she still wouldn't go in. I tried separating the stalls and that did not work either. I also tried deleting the fence in between and redrawing it with no change. Finally, I made that one stall just one tile wider and she went in.
When I changed the position of the stall and placed it across from the other one, it worked.
So there definitely seems to be something wonky about placing this particular stall right next to another one. Very strange indeed!
Edit: I've done some more testing and found that if I add the rooms in build mode from the map view and before I move any sims onto the lot, then horses will go into both stalls without a problem. After moving sims into a lot, if I go to map view and enter build mode to add the rooms, horses won't go into either one.
