Uploading new SSL certificate in 2018.1 On-Prem results in :
This site can’t be reached
The web page at
https://<On_Prem_IP>/controlpanel/actionfiles/sslcert.php might be temporarily down or it may have moved permanently to a new web address.
Release: AGUE5099000-2018-Agile Central-Unlimited Edition SaaS-50 User Pack
Certificate and key have been generated by Certbot and both files had .pem extension and that's incorrect .
According to CA Agile Central Control Panel 2018.1 documentation , chapter SSL Certificate :
"(...) Generate or purchase your certificate (this should be a single root or unchained certificate) and copy your certificate (.cert) and key (.key) files to the CA Agile Central server. (...)"
After having changed the extensions of certificate and key to .cert and .key one was successful in uploading the certificate .