I receive the following error trying to enter the Document Definition Editor:
"Document Definition is already being edited by %username%"
The issue gist is the Project copy you are using contains non-finished editing of the Document Definition. So the Document Definition is now locked.
The solution is to login in on behalf of the user who is said to be editing the Document Definition and publish it or discard changes.
As a workaround, please try to duplicate the Document Definition and work with a copy.
One more workaround, please try deleting a file locking the Document Definition (with ".checkout" file extension) in the local project “Templates” folder. Then update the project on the server with a local copy or re-upload the project to the server.