So couple of possible things.
1. Are you perhaps on Comcast internet service? I know their default servers have some kind of security protocol that can cause Design’s cloud connection to kick back that error, so I imagine some other ISPs might have the same problem. If you go into your computer’s internet/network settings and change your default server from Comcast to either Google or Cloudflare’s public servers, it may resolve the issue.
2. If the above fix doesn’t resolve it, try clearing your Design cache. You can find the app to do that in the downloads section of the website, and if you can’t find it (It’s pretty buried and hard to find) you can contact support and they can tell you where to find it. Clearing the Design cache is a fix for a LOT of problems that crop up over time.