I use chrome and I am also constantly having this problem. When I click the connection to delete it, the connection turns yellow and the minus icon pops up, but when I click on the minus it does nothing. I have to delete the whole user interface component to get rid of an incorrect mapping or if I decide to change something on the UI. I love Appery so far, but this has been quite annoying because it wastes a lot of time for a small change. I saw other people say that an undo button would be useful, this is very true because it would help me for this mapping problem if the delete feature were to ever break down again (ideally it wouldn't, but the undo function is useful for a lot of other things as well).
Edit:
(Just logged onto Appery in Firefox and it works on here).