Insufficient permission for System Manager

I am the system manager. I am not able to access an event created by an employee and allocated to another employee.

I get this error:

I can access the TODO and assignment for event doctype.
(allocated to is not my email address)

What might be causing this ?

Yes I am also getting problems in this.

We are also including all necessary employees in the event participants list but still they are not able to access the event.

Did anyone find any solution of access permission of events?