I too have this problem, I am trying to play 1. Ke7# but it doesn't work.
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I get message "Oops! The move you have made is not a legal move. Please make a different move." but I'm sure it's a legal move.
In any case, normally if you try to make an illegal move the piece just won't stay on the square.
In my case it allows me to move the piece, but when I click "Submit" I get the message