Recently updated iCloud Calendar has become unusable in Edge, Firefox and Chrome

To make sure we're on the same page, you're finding this happens in three different web browsers, correct?

Yes

Are you seeing similar results with any other websites?

No

Since it's happening with multiple browsers, we'd recommend testing from another Wi-Fi network to narrow it down.

I've tried different Wi-Fi networks and results are same.

Also, if you use any security, antivirus, or VPN software, it might be helpful to temporarily disable those and test to see if there's a difference.

I've tried that and results are same.

FYI. I've also received multiple error messages like this and I've clicked "Send to Apple".

And the diagnostics includes following line:

REPORTED ERROR TITLE

Maximum call stack size exceeded

Are you accessing iCloud Calendar on iCloud.com?

I am accessing iCloud Calendar on https://www.icloud.com/calendar/

Is your computer connected to a reliable Wi-Fi network with good signal strength?

Yes

Have you checked for any Windows updates or browser updates?

Yes, both Windows and browsers are up to date.