Hi turingtest2!
First of all, I'd really like to take a moment and thank you for all the help you've been over the years - I've read many of your support articles and guides, and you've really been a life-saver on more than one occasion. Thank you so much!
I'm having this same issue, and the steps you outlined above didn't help, I'm afraid. I even uninstalled iTunes and did a fresh install per your guide. Does it help at all to know that when this error message occurs as soon as I try to turn on the iCloud Music Library? It also happens as soon as I log in to iTunes when I've tried to log out and back in.
Another strange side effect, for me, is that iTunes switches to the "store" tab as soon as I launch it, instead of staying on the "library" tab as it has done for many years. I'm using iTunes 12.6.5.3 on a 64-bit Windows 7 machine.
Is there any chance that additional info makes you think of any other possible solutions? Either way, I really can't thank you enough for sharing all of your expertise with us all.