This sounds like it might be due to the DoS mitigation measures listed here, where your IP may be blocked:
And it sounds like you also suspect it was blocked. Did you get this IP recently, or was the server with this IP recently compromised? If this is a renewal, then this worked in the past, right? So did you recently change IPs or the like? Generally there's only a block like this if the IP was abusing Let's Encrypt's systems.
@lestaff, can you look and see if this IP is blocked?