SmartThings could not validate the endpoint configured in base_url

My HA is accessbile from outiside the local network (internet) and my SSL certificate have no problem (my browser tell me that certificate is valid), but SmartThings configuration give me the error in topic.
Trying to debug as specified here https://www.home-assistant.io/components/smartthings/, using curl, I get the error “Trust anchor for certification path not found”… what it mean?
Where I’m wrong?