I do have a Synology Diskstation with the latest DSM installed and I was trying to request certs for multiple domain names I own (which are banthon[dot]de, banthon[dot]com, banxx[dot]net (all via DD24 as registrar) and banthon[dot]net (via ChangeIP).
All of those domains are updated as www subdomain with my actual IP address daily via DDNS and I´m able to connect to the home page of all of those domains via port 80 and 443, which are actually hosted on an Apache on my Synology.
I was succesfully able to generate the certs for the first three domains via the DSM assistant, while it fails for the www.banthon[dot]net domain. The error message reported by the DSM assistant is
failed to connect to let's encrypt. please make your diskstation and router have port 80 open to Lets Encrypt domain validation from the internet...."
I SSH´d in to my Synology and performed a ping to those domain names and the LE servers, too and was able to successfully resolve/reach every domain, I also checked the domain config via https://intodns.com and they just seem to be fine and identical. Since the request of all domains except that one .net domain was successful the error message appears to be wrong by indicating there is a connection issue.
I´d appreciate if you could share your ideas what troubleshooting steps I can take to investigate further.
Does anyone know where on the Synology the logs are stored which may provide further details about the cert request to LE?
I used to have a startcom cert for www.banthon[dot]net which, according to CRT.SH, is still valid and also has a SAN entry for www.banthon[dot]net - Also, there is still an LE cert valid fot banthon[dot]net until April 30th. Could this affect the cert request attempt to LE for www.banthon[dot]net?