Hello everyone, same problem here. I've just updated Anki from 2.0 to 2.1.15 and it shows the same message mentioned above: “Error establishing a secure connection. This is usually caused by antivirus, firewall or VPN software, or problems with your ISP.”
However, I have no problems at all when using Anki 2.0. I can easily synchronize and upload text, images, audio... The problem is that syncing support for Anki 2.0 will end on Jan 1 2020 so adding new cards for all your devices will be a tedious task if synchronization does not work.
I already restarted my computer and waited some hours, but it didn't work.
My operating system is Windows 10 Home. Thank you very much for your help!