Encountering Popup of ‘Unknown certificate’ while Connecting to the Server Through FTP Client in Plesk Hosting

To enhance the security, we’ve configured FTP over SSL and removed plain text authentication in our Windows Server 2016 Shared hosting. 

So, if you are connecting first time through FTP, it will ask you to allow a certificate, as shown in the image below -

Select the Always trust certificate in future sessions option and click on the OK button.
This will encrypt all your FTP connections.