Unable to create or edit Events and getting error "No access to field EventWhoIds. Either the field was removed from the entity or access to this field was removed"

This issue occurs When Shared Activities is partially enabled and this causes the background migration job not to be completed successfully causing the access error. This affects all users regardless of profile and persists across browsers.

The issue can occur based on any of the following senerios

- Error appears when saving a new Event or Task with a Contact in the Name field

- Error also appears when editing an existing Event or its related Contact

- Issue affects all users/profiles including System Administrators

- Events created without a Name (Contact) save successfully

- Issue persists across browsers, incognito mode, and cache clears