Somewhat surprisingly (given the creator's regex chops) capital letters are not accepted. My guess is that your autocorrect changed the move to A2a3.

Bad news if you're almost done with a game and enter a move with the wrong case.

I'm on PC (firefox) and can reproduce it, no capital letters in my input

Hm. I don't get any errors unless I use a capital, and I get an error every time I do.