I don't have the steps to reproduce, but I guess it's related to the user session (after the session ends).

Potential steps to reproduce

• Access to the admin interface (view a content item)
• After the session ends, user is redirected to the login page
• Log in again

Result:

• User is redirected to admin/notifications/count
• Page displays

   {"pending":0,"total":0} 

Expected result

User is redirected to the content item he/she was viewing before being redirected to the login page