I've tried the following things support has told me to do:
- Under the "General" tab, please delete ALL browser history
- Under the "Security" tab, please move the slider to the lowest possible settings or no higher than "Medium"
- Under the "Privacy" tab, please move the slider to the lowest possible settings or to "Allow All Cookies"
- Under the "Advanced" tab, please make sure that "Allow software to run or install even if signature is invalid" is CHECKED.
- Make sure that all SSL and TLS (EXCEPT SSL 3.0) are CHECKED.
- Changed the compatibility mode to Windows 7
- Opened ports 54992 - 54994, 55006 - 55007 and 55021 - 55040
- Running game as admin via Steam