FINALLY I FOUND IT.
I am using Bitdefender Internet Security and one of the settings regarding SSL traffic was the reason.
I turned that setting "off" and ZenMate 's encrypted secure connection did work again. That setting must have blocked all encrypted traffic with ZenMate.
I don't know however how this setting has become "on" as the same happened to my wife's notebook who uses another version of Bitdefender Internet Security. An automatic update might have been the cause.
P.S.
(SSL (Secure Sockets Layer) is a standard security technology for establishing an encrypted link between a server and a client—typically a web server (website) and a browser; or a mail server and a mail client