The content.dropboxapi.com servers are being served with a valid certificate (from DigiCert), so it sounds like there's an issue with your local configuration preventing your system from validating that. You'll need to make sure your local trust store contains DigiCert.
Thanks for the answer, indeed, in my home network it works fine. In my professional environment, suddenly, I got this issue from the day to another. I guess the Company Firewall + Antivirus server must filter that.
Is there any support page, that serves this kind of information such as firewall rules, etc? I can address that to the tech support.