I have traefik deployed and directed to the host. All containers with third-level domain routes are working correctly.
I want to deploy a container so that it responds to mysite.org, but I get the error:
traefik | time="2020-09-15T06:35:27Z" level=error msg="Unable to obtain ACME certificate for domains \"<URL>\" : unable to generate a certificate for the domains [<URL>]: error: one or more domains had a problem:\n[<URL>] acme: error: 403 :: urn:ietf:params:acme:error:unauthorized :: Cannot negotiate ALPN protocol \"acme-tls/1\" for tls-alpn-01 challenge, url: \n" providerName=letsencrypt.acm
If I specify URL = site.mysite.org - then everything works correctly
Do you have something like CloudFlare (or other proxy between LE and your traefik instance) that might be filtering requests?
If nothing of the above helps try doing the same with let's Encrypt CLI from the same host to prove that this is not infrastructure problem and cert generation works without traefik and with LE.