I'm using the following method to change the ownership of an Object:

from AccessControl.Owned import Owned

def changeOwner(pObject,pUserFolder,pUsername=''):
userObject=pUserFolder.getUser(pUsername).__of__(pUserFolder)
pObject.changeOwnership(userObject)
Owned.changeOwnership(pObject,userObject,recursive=1)

And I call it as follows:

<dtml-call "REQUEST.set(objectValue,
REQUEST.resolve_url('www.myurl.com/myFolder'))">
<dtml-call "changeOwner(objectValue,acl_users,'tester')">

The above solution changes the ownership of the folder and the objects inside of it (I can see it on the manage interface). My problem now is that I get the following error whenever I try to edit an object that I own:

Site Error
An error was encountered while publishing this resource.

Unauthorized
You are not authorized to access indexObject.

Why do I get this message if I own the object which I want to modify?
It doesn't happen if I create a new object rather than transfer the
ownership. What am doing wrong?

Thanks in advanced,
Josef.