Morning, @MikeMcQ !
Okay, understood. Went ahead and restarted both the server, and when that didn't work, rebooted the whole machine. Still getting the same error seen above.
Double checked my port forwarding and as far as I can tell, 80 is passing through. I'm wondering if I'm getting bit by the LetsEncrypt geoblocking bug from earlier this year? I haven't tried renewing since this update.