No:
What IP addresses does Let’s Encrypt use to validate my web server?
We don’t publish a list of IP addresses we use to validate, and these IP addresses may change at any time. Note that we now validate from multiple IP addresses.
But if you're having trouble figuring out what's happening, one useful technique is to just run a packet capture (using tcpdump or tshark) on port 80, while you request a certificate.
That will show you the full request/response bodies and will hopefully let you identify what exactly is happening.