When trying to schedule an appointment in Student Success & Engagement, a warning message displays that "There was a problem saving the event".
To resolve this, make sure all attendees are assigned a primary email address at Student Success & Engagement AdministrationPeople & RolesPeople on their Person Details*.
If missing, in order to resolve this quickly, the primary email address can be set on the Override menu, further explained here.
*By design, SS&E will always use the primary email address for everything except when explicitly sending a message to a "different"/personal email address.