[SalesForce] Unable to edit Public Calendar Event

One of the user is not able to edit some of the public calendar events.

Here is the error message when the user is trying to edit the event.

Insufficient Privileges You do not have the level of access necessary to perform the operation you requested. Please contact the
owner of the record or your administrator if access is necessary. For
more information, see Insufficient Privileges Errors.

Click here to return to the previous page.

I looked into this link but it didn't solved my issue.

Here are few things that I have things that I've checked

1. User has FULL access to public calendar
2. Related object (Event's Related To) has Public Read Write and Grant Using Role Hirearachy is checked
3. User is subordinate of Related TO Owner so written a sharing rule on Related Object which gives Read Write access to all Related TO records in Organization.Also clicked on Recalculate Button once sharing rule is written.
4. User has Modify all permission on Related To Object.
5. User has Edit Events and Edit Tasks Permissions

Also looked into this link

Please let me know if I missed anything