A family of Microsoft products that enable users to capture, organize, and reuse notes electronically.
I believe I did finally figure this out. I never received anything helpful from Microsoft about it though.
The fix: Log in to your Outlook email account. There should be an email there (I forget the exact wording) that says Microsoft periodically requires you to verify that you are still using your email account. The purpose is to prevent your account from being used to send spam. Do whatever verification it says to do.
Why it works: Part of the process of sharing a notebook is that an email gets sent from your outlook account to the person you are sharing with. I never use my outlook email and never log in to it. So because I ignored that email my account was frozen and it could not send an outgoing email. If you can't send an email then you can't share. I was able to share my notebook again as soon as I unfroze the email account.
I hope this helps. I wish Microsoft would fix this because sharing documents should not be linked to email like this.