This line, that I added, seems to have fixed the issue that some people were having.

If you have slots on another world that is not in your main server folder, but you still kept some on your current world, the plugin will no longer freak out, so instead of it not loading one slot, and disabling all slots, it will instead just skip loading that slot.

I would also check your spigot/paper/purpur server folder to make sure the world the slots are in match with the name listed in the slots.yml file.