I cannot say what's wrong there. The data/connection might be invalid, in some way. Can you try to navigate to the document in the Document Revision screen and remove the connection there, from under the Objects tab? If you cannot navigate you can open the Document Revision screen and search for the document using the Doc No, and then disconnect.

It might also be a bug, but my guess is it's the data that has become invalid in some way and for some reason. Do you know how the document was created and attached in the first place?
 

@Mathias Dahl thank you for your reply.

the document revision is valid and can be open and view. and this problem looks like is due to permission set. I can easily detach and attach in testing environment, but not in production. 

let me talk to our technical guys.

Find the document in the Document Revision screen and see towards what objects it's connected. Is it directly connected to the engineering part revision?

@Mathias Dahl it is directly connected the object Engineering Part Revision window.

Is it the same for all documents? I anyway cannot explain it. It does not sound like a permission issue, but who knows... Let us know the cause of the problem if you figure it out. It does not sound like a bug.

By the way, do you have any custom events for the document object connections? (LU/entity DocReferenceObject)