I have installed 21H2 using the Windows Update Assistant since posting and the pop-up still pops up - no change.
This issue is a Microsoft inclusion in their May Windows update. It never happened before.
I get a panel pop-up for each Internet items minimised to the taskbar on opening from Sleep. The panels are black for a few seconds then go white and say "just a moment" then disappear.
The fact they disappear so quickly infers to me that my PC is perfectly OK. They do not appear EVERY time I open from Sleep.
I tried the network adapter change - it never cured it.
If it is Microsoft's program problem, then Microsoft must put it right. I am not going to risk serious surgery on my PC just to get rid of two panels that simply pop-up and disappear.
It is obviously a balmy Microsoft attempt to alert users that there is no Internet connection and not an error on the part of users computers.
It does not allow for the few seconds that a PC may take to respond from wake-up.
These panels do not show when opening from shutdown, because there are no live programs on the taskbar.
The error panel is obviously activated on Internet closure and re-activates immediately the PC is switched on, which explains why it only takes a second or two to disappear once the Internet becomes live.