User can't delete / change an (shared) object from an open worksheet.
When changing ownership on a server object, from one user to another, that object does not show up automatically for the new user. The new user can find the new object in the Repository, and can add it manually, but it doesn't show up on the same place as for the former owner.
files is corrupt or the user hasn't the permission to delete an (shared) object
When an object is created by a user in the client, it is placed on a sheet, which will become its location. When the ownership of the object is changed, the location of the object is not.
This is because the former owner added the object to a sheet that the new owner does not have, hence it cannot be decided where the object should be visible.
Open the QMC and go to register "Documents". Chose the effected file.
Search for the object in the list and delete it. The object won't show up for now.
Open the Repository and add the object to a sheet when in the document as a client. This will position the object on that sheet and have its position saved to that new location.