Can't log in to Music/iTunes store on MacBook When Apple TV+ came out, I wasn't getting the one-year offer as I should have and got the advice from Apple's online chat to try just completely logging out and back into my Apple account. 

I logged out of the apps and iCloud in Settings, restarted, and logged back in, in Settings. Now I can't log back into Music, TV, or Podcasts (although Books and the App Store are just fine). I go to the Account menu → Sign In..., put in my username and password, and the login window disappears as if it's going to log in, but nothing happens in Music or TV. Podcasts actually says "something went wrong during authentication (0)". I don't get a two-factor authentication notice, and if I go back to the Account menu, it still says "Sign In..." instead of showing my account name and balance. Given the affected apps, it seems to be an issue with the iTunes store. Indeed, in Music I also (usually) can't get into the store - if I click on it, it may rarely work, but mostly it will just spin forever, or give me error 11556.

Interestingly, making a test user, I can log in. I figure it has to be a plist or another setting that's been corrupted, but would be fresh on a new user. Going to the Library preferences, I deleted the iTunes store plist, and now I can see the Store, but still can't log in. 

What plist should control iTunes login? Or have an idea where other Apple Media Services files live? 

I would exhaust other options before a reinstall.
I just got an email from the engineer I had spoken to on my last call to Apple Support and it turns out there were some server issues on their side. They asked me to try logging in again and lo, and behold, now it worked perfectly.