If everyone in the area had died, there would (more or less by definition) be no obstacle. If there were neighbors still alive to object, it depends on whether the previous inhabitants had owned the land. If they had been only tenants, the actual owner (perhaps a local lord or monastery) would be likely to come along and put a stop to the squatting, at such time as they weren't being themselves overwhelmed by the plague. If the previous inhabitants had owned the land, though, there might be a question of whose business it would be to object. If someone with a reasonable claim to owning the land showed up, he could file a writ of novel disseisin at the assizes and get the courts to look into it. If no one at all had any claim to the land, I'm not really sure what would happen. The "squat-to-own" approach isn't exactly legal (at least, until it succeeds), but in the chaos of the plague I wouldn't rule it out entirely.