iTunes repeatedly demaniding I sign in to iTunes store, but rejects my valid password
Recently iTunes 12.9.5 on my old MacBook Air running 10.14.6, repeatedly demands that I sign in to the iTunes Store. I don't use the iTunes Store. Worse, it rejects my valid appleid password. For example, I signed into this site without an issue. Even worse, it is a modal window that cannot be dismissed. I must force-quit iTunes. But it comes back eventually after relaunching.
Based on a suggestion in a 2021 thread of the same complaint, I ran Malwarebytes, no threats found.
I have a very large music library, 54,000 items. I use iTunes to organize the library, and I listen to music on my MBA, but also use it for synchronizing playlists on my iPhone.
iTunes has become completely dysfunctional. If I can't stop this behavior I will start looking for 3rd party solutions if there are any, I am sick of this dysfunctional software.
Any help deeply appreciated.