This looks like Let's Encrypt's DoS blocking.
Is the IP sending the request recently acquired by you? If not, you should double-check that there isn't anything unkind running on the server, as generally blocks like this only happen if they're attacking Let's Encrypt's infrastructure.
@lestaff, can you look and see if this IP is being blocked?
@Simon89, you may need to post the IP that's trying to make the request.