This was what I was told by PTC support for arbortext editor.
In the error window it will tell you in brackets the number of the offending font (129,0x0081).
You will use the first number to search for the bad font. This window will also tell you where to locate the bad font. If it says something about filename.style you need to look in the style sheet. If not it should tell you show you a path to the bad file.
How this procedure works is that you give the character an atribute of "findit", then you search on that attribute. If you do not do this then you will not be able to search for the bad font asthere is nothing to search on.
Open the document or fosi in Arbortext Editor. At the Arbortext Editor Command: line enter
$FINDIT=chr(129) and hit the Enter key Now enter (find "$FINDIT") you may not need the quotes.