Okay, this gets more and more bizarre. I sent a support email to Apple and basically they said "...it seems that someone else associated your Mac with their Apple ID. That's the reason your purchases won't download."
Yikes! My wife and I live alone- and we have exactly one and only one Apple ID. Would somebody have needed physical access to my iMac to do that?
Support then said "You can view which devices or computers are currently associated, remove unused devices or computers, and see how long before they can be associated with a different Apple ID from the Account Information page in iTunes on your computer."
Okay- so I logged into My Account in iTunes. On the list of associated devices, my own iMac is NOT on that list. But I am logged in under the same Apple ID I've had for years on that iMac.
So my big question right now is...if my iMac (that I've owned for 5 years) has somehow been dis-associated from my Apple ID, how do I find out what Apple ID it is associated with?
Thanks gang for trying to help me unravel a mystery.
PS- Here's an Apple Tech article on iTunes associations, but it doesn't really go beyond what we've discussed above: