Thanks for the update. The same thing was happening to me, and I found this thread looking for the solution. I've followed your tip of unchecking "Show Apple Music" and if the dialog box returns I'll add a follow-up.
By the way, this problem is not unheard-of, as we see in this search:
https://encrypted.google.com/search?q="icloud+music+library+session" I never would have given any thought to the "Show Apple Music" setting. Thanks again!
