Many websites not loading in El Capitan

Dear community,

Many (not all) websites are no longer loading in Safari, Opera, or Chrome under El Capitan (10.11.6 (15G22010)). A website that loads in one of those browsers loads in the others, too, and vice versa, wich seems to indicate system wide problem. Opera and Chrome name the error (NET::ERR_CERT_DATE_INVALID) pointing to date and time set incorrectly on the mac, yet time and date on my mac are in fact correct.

Error messages:

Safari:

Safari can’t open the page “https://www.flaschengeister.berlin/cart/overview” because Safari can’t establish a secure connection to the server “www.flaschengeister.berlin”.

Opera:

Your connection is not private

Attackers might be trying to steal your information from www.idealo.de (for example, passwords, messages, or credit cards).

NET::ERR_CERT_DATE_INVALID

Reload

Help me understand

www.idealo.de normally uses encryption to protect your information. When Opera tried to connect to www.idealo.de this time, the website sent back unusual and incorrect credentials. This may happen when an attacker is trying to pretend to be www.idealo.de, or a Wi-Fi sign-in screen has interrupted the connection. Your information is still secure because Opera stopped the connection before any data was exchanged.

You cannot visit www.idealo.de right now because the website uses HSTS. Network errors and attacks are usually temporary, so this page will probably work later.

Chrome:

Your connection is not private

Attackers might be trying to steal your information from www.idealo.de (for example, passwords, messages, or credit cards). Learn more

NET::ERR_CERT_DATE_INVALID

Any idea?

Thanks so much

[Edited by Moderator]