@Damo: Based on our discoveries in this possibly-related issue: Might you have configured your server to limit the enabled cipher suites, either through group policy or through software like "Nartac"? The Let's Encrypt API endpoint recently changed from using RSA to ECDSA, which most systems should support but some over-zealous "hardening" might have disabled ECDSA cipher suites on your server.
I'll also second the previous recommendation to make sure you're on the latest version of win-acme. Or you might want to switch to the newer simple-acme fork. But I don't think that'll help on its own, until you sort out why you can't connect to the Let's Encrypt API. Once you can connect to it, the error message should change if port 80 connectivity inbound to your system is still a problem, as some posters above think might also be a concern.