Clear your browser cache and history. Skipping this step is why most errors happen.
Clear Cache → Close Browser → Reopen → Log in.
ATTENTION: Do not clear cookies. Cookies are what keep you logged in to websites. If you delete them, you'll get logged out everywhere and have to type in all your passwords again. Clearing download history won't delete your files. It only erases the list of things you downloaded. The actual files are still safe in your Downloads folder.
Safari bundles history and cache together, so it takes two steps if you want to be thorough.
To clear history and cache:
To clear your download list: