Software Update failing on High Sierra: (NSURLErrorDomain error -1012.)

When I try to run Software Update (on High Sierra), I get the following error:

"An error has occurred / This operation couldn't be completed. / (NSURLErrorDomain error -1012.)"

I've been trying a few solutions I've found here and on google, but I don't want to make the problem worse by trying something more complicated that I don't fully understand, so guidance would be appreciated.

I noticed I've had a few 'network' related errors popping up in different places the last few weeks, so I think this is another one, and I suspect they are all part of the same problem. Hopefully these can give you a clue:

-screen sharing (with Apple support) won't connect any more. it says it's connected but they see nothing.

-adding new accounts in SystemPreferences>InternetAccounts fails to load remote web content.

-when I connect new iPhone 15 - 7.0.3, high sierra will prompt to install update to support the device, but the download fails to establish after I click to install.

-High Sierra installer 1) downloaded from Mac App Store or 2) from Recovery Install, both fail to connect to servers during attempted installations.

-and now software update.

As a test, I installed a clean High Sierra on another drive (same Macbook Pro) and 4 of those 5 issues don't occur on the clean system. I can't test screen sharing or I might be 5 for 5.

I tried reinstalling High Sierra on top of itself (on the problem system), but I still get the same errors afterward.

I tried a different internet connection, but no change.

I have NO VPN.

I have tried with built-In firewall disabled.

I have Little Snitch, but it's disabled.

Ideas?

Help?