iCloud encountered an error while trying to connect to the server

I had the same symptom but in the web version of iCloud.

One way I found to fix it was disabling IPv6 in the wifi adapter settings. You can do that in the Control Panel in Control Panel\Network and Internet\Network and Sharing Center\ then click on the network adapter, then click Properties and uncheck the IPv6 item. Not ideal because it's the way of the future and provides improvements over IPv4.

For me another fix was found by comparing two systems, one that had the error and one that didn't, and it was to clear the "Automatically detect settings" in Internet Options. Open Internet Options (Control Panel\Network and Internet) go to Connections tab, LAN Settings, and uncheck "Automatically detect settings". Oddly enough, I was able to re-check it and it also works, so it must have just cleaned up something bad. No idea 🤷‍♂️

I disabled Intel Killer Performance service but it made no difference for me. So far only disabling IPv6 in the Wi-Fi adapter settings fixes it.

Forgot to mention that if you use iCloud via browser, Firefox still works for me. It's just the Chromium-based browsers that don't work: Google Chrome and MS Edge Chromium.

My bad, I made assumption that shutting the Killer Network Service service off was the same as disabling, but it is necessary to disable startup for the service and logout/login or reboot to have it affect network settings. Then iCloud site works. Thanks for that.

It also found I could keep the service on, but go into the Killer app and toggle the Killer Prioritization Engine to Off. No reboot required, and it persisted after a restart.

In my case really don't think I need that app messing with how apps use my network, so I just uninstalled it.

Unfortunately, the Internet Options fix did not last, and after a week or so I started getting errors from icloud.com again. So, for now I am going with disabling IPv6 in my wifi adapter settings.

Can confirm that I can't connect in Chrome using WiFi. It works when I disable my Intel "Killer App" wifi and connect via LAN.