I have virtually the same occurring, so is our confidential information at risk and is there no answer?

Hello namwob-43Bmth! Thank you for participating in the Apple Support Communities.

We understand that you’re getting an error when trying to connect to your iCloud account with your Calendar app. We'd like to look into this with you to see how we can help. 

Let’s try the following: 

• Restart your Mac: Log out, sleep, wake, restart, or shut down your Mac

• Update your date and time settings to Set Automatically: Set the date and time on your Mac

• Connect to a different network to see if the issue persists

• Start up in safe mode. This can help determine if this issue is related to software loading when your Mac starts: Start up your Mac in safe mode

Make it a great day!

namwob-43Bmth wrote:

have virtually the same occurring, so is our confidential information at risk and is there no answer?

Calendar can't verify the identify of the server "p31-caldav.icloud.com".

The certificate for this server is invalid. You might be connecting to a server that is pretending to be "p31-caldav.icloud.com", which could put your confidential information at risk.

Same occurring what?

If you have iCloud problems sign out of iCloud and sign back in...

>System Preferences>AppleID>iCloud

Sign out of iCloud on your...Mac

I see no issue reported on the server—https://www.apple.com/support/systemstatus/

As @ leroydouglas suggested first then if still an issue >>

For peace of computer and mind >> Suggest downloading from a Respected ASC Contributor the application Malwarebytes for Mac. It is free or paid for added features. Run the Application and it should remove the malware / adware. Once done, restart computer and test.

There are no known Windows-like Viruses in the wild that self replicate and affect macOS, because of the underling UNIX  Foundation and Permission Limitation. 

There are Malware-viruses and Adware-viruses that does affect macOS and are often times downloaded as part of an Application from Third Party UnTrusted Site and get installed along with the Application.