Hi I have same problem of this:
I have access on 443 and 80 in my subdomain
I have last version of DSM DSM 6.2.2-24922 but not fix.
2020-01-29T13:32:05+01:00 Sauron synoscgi_SYNO.Core.Certificate.LetsEncrypt_1_create[13252]: certificate.cpp:973 syno-letsencrypt failed. 200 [new-req, unexpect httpcode]
2020-01-29T13:32:05+01:00 Sauron synoscgi_SYNO.Core.Certificate.LetsEncrypt_1_create[13252]: certificate.cpp:1392 Failed to create Let'sEncrypt certificate. [200][new-req, unexpect httpcode]