If I remember right, the xref was new in 6.5. maybe they changed some things behind the facade in newer versions.
If you have a computer with 6.5 (or perhaps also 7.0), I would reccommend to
- open it there and find out, which files were placed by hotlink and where they are placed in the floor plans on which height and so on.
- maybe place some hotspots for easier placement
- then delete the hotlinks,
- save the file (new name!) in 6.5
- open it again in 8.1 and
- place the hotlinks again
ok, double work, but the results should be good.