"​Recurring appointments whose recurrence is the last day of the month cannot be migrated."  This error occurs because of the differences in the way recurring events are handled between Google Calendar and Outlook Calendar.

Resolution:

In Outlook, you can set a recurring appointment to occur "on the last day of every month". In OWA, you can create an event on the 31st day, and it will appear on the last day of the month for months with less than 31 days.

When you share an Outlook Calendar Event scheduled on the last day of every month to a Google Calendar, Google will accept the event and it will display on the last day of every month. When accepted, the Google user will receive the warning: "This event has a recurrence rule that cannot be edited in Google Calendar".

Even though the event displays properly in Google Calendar, it cannot be migrated back to an Exchange or Office 365 calendar, because of the way Google restructured the way in which the data is managed.