Dear All,
We have investigated this issue and concluded that it is a defect. It was quite easy to reproduce, all we need is having 2 DWGs with the same name placing at 2 different locations.
After we attach one of them, then detach it from the plan, attaching the other one from another location will result in the error message. Unfortunately, if you detach an Xref it doesn't get deleted in the background, and when you try to attach a new Xref with the same name but on a different path, it finds the previously attached one and fails. I entered it as defect DEF-3722.
Thank you very much for your understanding, and I am very sorry for the inconvenience!
Best regards,
Minh
Minh Nguyen
Technical Support Engineer
GRAPHISOFT