Michael Ward5,
I can't say for sure if this was the solution to my issue but I came across your post the most recent time I was trying to enable iCloud Music Library, and did find and delete the few tracks in my library that came from my wife's iTunes purchases (though Family Sharing), and now things seem to be working again! I was even able to sign out and back in, and to re-download those tracks, and run an update of the library and things seem to continue to work.
It's also entirely possible it was just a coincidence, or that Apple did something behind the scenes to my account. I had also spent an hour or so on the phone with them yesterday trying to work through the problem, and at the end it was said that my case was being referred to the engineering department and that I would get a call back on Monday with what they found. So I half wonder if it was sent over there and some engineer saw something with my account and just fixed it. Either way I'm very happy that things are working again (I did strangely end up with a few hundred duplicate tracks, but was able to use the "View > Show Duplicates" feature to weed those out pretty quickly. Anyway just wanted to update!