Using a wax ring was perfectly fine. If you used a horned ring THAT might be the reason it is leaking.
Also, the neighbor could of plunged the toilet and not make you aware of the situation. This can blow a wax ring out if the clog is beyond the trapway of the toilet.
Post pictures when you pull the toilet; the condition of the wax ring tells the story; if you used a horned ring then it most likely will pull up with the toilet.
Getting the visual on how far that ring compressed if it went too far will tell you what caused the leak.