Mail > Connection Insecure

I cannot access 1 of my 2 Google accounts, on Mail for Mac!

I keep getting: "Connection Insecure, Failed to verify the server certificate. This could be because of your network configuration or your proxy settings".

I don't have any anti-virus software, nor plugins etc. and I deleted 1 Certificate from the Keychain, restarted the Mac, but nothing works. And all 'Veri' Certificates are set to "Use System Defaults".

I strongly believe (in my casse) it has nothing to do with proxy settings, because my other Google account works fine on Mail for Mac and I can access both Google accounts on my Mail for iPhone, or a browser on the laptop (but it's a pain)!

In the end, I went to Security on my Google account and revoked the 'Trusted device', but still nothing works!?

I also deleted the account and tried to add again and no luck :(

What else can be done here, please? TIA.