Using Let's Debug (see here) to test connections to your server shows you have an AAAA record in the DNS for a private IP. You should change that to a public IP or remove the AAAA record to use only IPv4.
I don't know that this is causing your original problem but the error message is not very informative. It is something you need to fix to get an LE cert in any case.
EDIT: Also, your DNS config has some problems you should address. See dnsviz report