iTunes on Mac can't connect

Thanks for trying and for applying more human intelligence than the older reply I referenced. As it happens, however, none of those ideas is helpful. What actually fixed it was rebooting MacOS so something needed resetting and a really helpful answer would identify what that was and how to reset it without a reboot. I tried stopping and starting the LAN connection (Ethernet), but that wasn't it.

Interesting suggestion that it might be Malware, but it wold be odd Malware, I think, because the problem went away after a reboot (and that has happened before). It would also be slippery Malware because I am running Kaspersky AV. I'm not running a VPN and if I were I'd be surprised at it generating an intermittent problem like this.

The point about the system clock is a good one, but my system clock is automatically set to an internet time service and is reliably close to the correct time. If faced with a genuine Internet connection problem I would reboot the hub/router/broadband modem and in general that is a helpful suggestion. However, I had tested that already by accessing the iTunes store from an iPod and from another Mac on the same LAN and accessing other Internet services from the Mac with the iTunes problem. All of those worked so I didn't believe the router could have contributed to the problem. (Yes, there could conceivably be a malfunction that could have had that selective effect, but it would be way down the list of possibilities.)

That's an interesting idea, I'll give it a go when and if. I have several times solved other problems by killing the DNS responder (I usually use Activity Monitor rather than Terminal), but I've never tried explicitly clearing the cache or seemed to need that. Of course if it's a DNS problem then no connection would have been made in the first place and the diagnostic message that says the connection was unexpectedly terminated would be lying - but inaccurate failure messages are endemic to Apple (and probably other) software so maybe never mind about that.