I am having the EXACT same issue as dbvirago. I am able to share individual pages within my notebook. But when I attempt to share the entire notebook, I get the same red box with the error text "! Failed to share notebook". See the attached picture. Below are the steps I am performing:
Open the notebook you want to share.
Click the More actions button (three dots).
Select Share notebook from the drop-down menu.
Enter at least one email address (or select at least one contact) and set the permission level.
Click Send Invitation.