I think I might know what you were talking about - I had a similar issue at first. Is it a problem with the drop-down menus on "destination world" and "current world"? You have to hit the "confirm" button first after selecting the destination world, and only after confirming can you select your current world (it doesn't appear in the dropdown before clicking confirm). I apologize if that sounds confusing, it was a bit weird when I did it, and I'm not sure if you're having the same issue.

edit: OH nevermind, I saw that you'd solved it!