an error has krept into the database that makes up a Protool project (a "ghost" maybe ?).

To clear the error do as follows:
When the cross-reference is open, hold CTRL pressed and RIGHT-click anywhere with your mouse.
A menu will appear.
Select "scan all log-error".
When the scanning has finished, hold CTRL pressed and RIGHT-click anywhere with your mouse to get the menu again.
Select "repair all".
This will almost certainly fix the problem.
我也遇到过同类问题,照上面做就能解决.