In environments which have migrated from Exchange 2010 to 2013 (or later), and where public folders have been decommissioned, you may encounter the following events in the Application event log:
With the removal of the public folders database, not all objects referencing the database have been adjusted. You can manually fix the issue by clearing the reference to the deleted public folders database, using ADSI Edit.
As the change is in AD, clearing the values should only be done on one DC. The change will be replicated to all DCs and all Exchange servers will pick this up.