Thank you very much for the report and I am very sorry about the issue!
I was unable to find any similar report in our database, so I am wondering if it's possible for you to share the project plan PLN file and the DWG file with us for further investigation? Let me know if you need a secure place to upload the file also.
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!
We are having the same issue however we have cleared our ref manager and renamed our DWG files. Despite this we get the same message and are unable to attach a previously xref'ed file even if it was 'saved as' under a different name.
Is there a work-around to be able to address this issue and get the DWG file attached?