Hi technophobe2021,
In that case, we'll want to isolate the potential reason for this error message you're seeing. It may be due to a networking communication issue, to some unexpected software issue, or even an issue with the iCloud servers on our end.
To test for a network issue, the easiest step is to try your device on another network. Since you're using an iMac computer, it may not be possible to easily switch to another Wi-Fi network. If you're unable to join another network, let us know if your iMac is able to reach our iCloud website using the Safari web browser. Are you able to sign into iCloud.com directly? This can help confirm that your computer is able to communicate with our servers.
If you're able to sign in on the website, let's ensure that your iMac has accurate system time in the Time & Date preferences. When accessing secure content online, it may be critically important that your computer have accurate system time. In order to do so, head into System Preferences > Date & Time. If the option is enabled to set the time automatically, you'll want to temporarily disable this option for about 20 seconds, then turn it back on, and verify that the date, time, and time zone are accurate.
The following guide has detailed instructions. Let us know if you have any questions:
Set the date and time on your Mac - Apple Support
If the time is accurate and yet the same issue persists, the next step will be to start your iMac in safe mode, and then attempt to sign in again. Safe mode is a great way to resolve unexpected behavior, as it prevents login items that may be causing an issue from loading when you start up your Mac.
This guide will walk you through the process of starting up in safe mode, and testing from there. It's very straight-forward! Simply shutdown your Mac, power it back on, and then immediately hold the SHIFT key as your display turns on.
Continue to hold until you see the Apple logo appear. If the issue is fixed in safe mode, simply restart your iMac to start up normally, then confirm that it remains fixed, and that you can see all of your calendar events. If the issue returns when you start up normally, you'll want to test your login items by following these instructions:
Remove login items to resolve startup problems on your Mac - Apple Support
We're here for you if you've got any questions.
Take care!
