Hi NMP!

Are you using Nord VPN?

If so, it looks like a recent update to NordVPN is breaking some website functionality. It isn't just Calendly - however we have heard the same issue from multiple users. We have been in contact with NordVPN’s support team and they are aware of the issue.
 
Here is an article that goes into a little more detail: https://nordvpn.com/blog/how-to-fix-nordvpn-blocking-websites
Their workaround is to try the following:

• Click the “Unblock” button, which should be visible while attempting to access Calendly, or edit your Threat Protection settings to whitelist Calendly.com
  IF THAT DOESN’T WORK;
• Turn off threat protection by clicking on the little shield icon on the left side of your screen and turning off the Threat Protection switch
  IF THAT DOESN’T WORK;
• Quit out of NordVPN completely
  • Note: simply turning off NordVPN doesn't fix the issue

 
You may also want to contact NordVPN about this too so that they are aware of the impact.