Mac App Store -- Unable to Check for Updates -- The operation couldn’t be completed. (NSURLErrorDomain error -1100.)

I am unable to check for updates using the Mac App Store on my Mac running High Sierra.

I get this error:

The operation couldn’t be completed. (NSURLErrorDomain error -1100.)

I have followed the recommended steps at https://support.apple.com/en-ca/HT201400

but the error persists. My time is set correctly etc.

I have even reinstalled the 2020-005 Security Update for High Sierra.

The App Store does open. I can even log into the Mac App Store and download and install past purchases on my Mac. But I CANNOT check for updates. Very frustrating.

Any ideas.