Every time I export a C3D drawing to ACAD (command: _AecExportToAutoCAD20xx), I get this "error message"
There does not seem to be a real problem because I can open the exported drawing in vanilla AutoCAD OK, (although I do still get a proxy notice..?).
A few other older threads about this reveal no solution or confirmation that this dialog is a bug.
I was getting this one time, along with the proxy entity warning too. Turned out there was one locked cogo point in the drawing that wasn't gettng exploded.
Credit where credit is due! Give kudos or accept as solution whenever you can.