My apologies for causing you to waste some of your time. The problem I’ve been encountering apparently wasn’t caused by any bug in GS.
Instead, it seems to have arisen from something dumb. I got a new iMac last month, and set it up with the Magic Keyboard that accompanied it. However, I much preferred my old extended USB keyboard. So last week I hooked that up to the Mac, and since everything worked fine initially, I didn’t give the changed setup another thought. I didn’t try using GS the first couple days after that, so when I did on Friday and GS was totally blocked by the error alert (but with none of its settings any different than usual), I didn’t associate the problem with the keyboard change.
However, when attaching my old USB keyboard to the new Mac, I merely moved the bluetooth magic keyboard a few feet off to the side. I’m not used to having one which has a little on/off switch, though, and neglected to turn off its power when I put it aside. But it appears that somehow the signal it continued to give out was breaking the connection between GS and PayPal. Now, after turning it off, and refreshing my eBay token, all the error alerts disappeared, and I was just able to smoothly list an item, as a test.
So, it looks like I’m back in business, after four nights of fitful sleep. The cause of all my frustration was a very simple and stupid thing. Computers are so complex, though, that a simple thing can mess them up in a big way.
It is a set of circumstances your other customers are unlikely to ever reproduce, but in case someone else has a connection problem like this in the future, make a note in your knowledge database that it could be caused by a conflict when two keyboards are active at the same time on their computer.
Arrgh! (Enjoy the rest of your day.) - Best wishes, Nick