Thanks for the replies. This was the problem: I've had the 1h forecast checkbox checked so it seemed like everything should work:
However when I clicked on it, it asked me to buy Windy Premium (which I already bought). I tried to restore my purchases and found two orders in FastSpring: one free (try-out period) and one paid. I've entered the order reference from the paid one but Windy said it's not valid. I've entered the free one and now everything works.
What's interesting is that the forecast was displayed by one hour:
Only the keyboard shortcuts were broken.