Many thanks for your reply.
According to your suggestion I've analyzed the sample config file coming from github (/etc/dehydrated/docs/examples) and I've modified the mine in this way:
WELLKNOWN="/home/e-smith/files/ibays/Primary/html/.well-known/acme-challenge"
#CA="https://acme-v02.api.letsencrypt.org/directory"
CONTACT_EMAIL=info@qbservice.it
HOOK="/usr/local/bin/dehydrated-hook"
API="2"
The trick was outdated "CA =" record
All other folder/scripts and apache/SSL settings were right.
Swapped to API2 too.
I've been able to perform correctly both test and getting trusted certificate.
Many thanks again.
P.S.: not using contrib because more or less all of mine letsencrypt install were done before contrib release and I never moved to contrib.