I'm very sorry to hear this. Tiki may be rebelling because the room where he usually goes to use the litterboxes is now closed off. Since he was checked out and is fine, it's has to be a behavioral problem. When I was painting my cat Starr started pooping outside of the litterbox because he was very unhappy about being cooped up in my bedroom with all of his siblings. Starr will still do this occassionally and I know that once they start it's a hard habit to break. I hope that you'll be able to find a solution to your problem. Tiki may not be the only one that is doing this. Good luck.