This means the issue isn’t in any way related to Homebrew-Cask, but with the vendor or your connection.
Start by diagnosing your connection (try to download other casks, go around the web). If the problem is with your connection, try a website like Ask Different to ask for advice.
If you’re sure the issue is not with your connection, contact the app’s vendor and let them know their link is down, so they can fix it.
Do not open an issue.