First try using a different browser. I recommend using the latest version of Google Chrome. Make sure you disable all of its extensions first.
If this doesn't work, try disabling any antivirus/firewall/Internet security application. Windows Firewall and Windows Defender are safe to leave enabled.
If nothing of the above works, please try using a different PC, ideally connected to the Internet through a different ISP. We had many cases where the PC setup was broken and using a different computer did the trick.
If the above information doesn't help, please go back to the previous page and continue reading.