I don’t know if anyone here knows how FreePBX is configured to request Let’s Encrypt certificates, I’ll post it here, maybe the error message is general and will make sense here too.
Whenever I’m trying to issue a new Let’s Encrypt certificate in my FreePBX isntance it gives me this error message:
You should take this to community.freepbx.org . Whatever way FreePBX issues certificates, /.freepbx-known/ is not part of the Let’s Encrypt/ACME protocol, so it’s likely to be a FreePBX-specific issue.
You should ensure that http://pbx.mydomain.com actually points to your FreePBX instance and is accessible on the open internet (use only port 80, and ensure any AAAA/IPv6 addresses are actually reachable). Apart from that, there’s not much to go on and the knowledge to answer it probably isn’t on this forum.