If you haven't already, try an "Incognito" window if you're using Chrome, or "In-private browsing" if you're using Internet Explorer / Edge.
For either, just right-click the shortcut on the desktop and you should see an option, then login as normal and see if it works.
However, it's likely to be a problem with their site (HTTP 403 means you / everyone doesn't have access), so you'll likely need the website administrator to sort the issue.
Hope that helps.