Sometimes if we are used to authenticating from ssl certificate to email, ftp or even to the plesk itself when the default certificate expires, depending on the device it can give us errors. To solve it, we issue a new self-signed certificate.
We enter the plesk as administrators.
We click on tools and settings.
We enter ssl/tls certificates.
We click on add.
We put the name of the certificate, for example Default2017.
We fill in the company name with our name and the domain with the server hostname, for example dns19650.phdns.es
We click on "Self-signed".
We go back to ssl/tls certificates and set it as default in plesk in all options depending on the version it will give us more or less.
It is recommended to use the hostname used for certification in all ftp, plesk, email, etc. services.