Let’s Encrypt made changes to their system on the 19th of February which now changes how they validate domains. They check things from multiple network points rather than a single location so if your DNS provider for instance blocks DNS queries from some countries they may see an error when they try to look up your domain.
Another user had a similar problem with worldnic hosted DNS (network solutions?) - I think this problem is teething issues with Let’s Encrypts changes and the issue will disappear. We have no control of the Let’s Encrypt service though.
If the problem persists and prevents you from renewing your certificate you should raise the issue on the Let’s Encrypt community https://community.letsencrypt.org/ as that forum is monitored by Let’s Encrypt employees.