I'v the same problem. After opening and closing a form (even without saving it) rebuilding app gives same error than you have.

Compiling d:\program files\microsoft visual foxpro 9\ffc\_base.vcx
File d:\...\microsoft visual foxpro 9\ffc\_base.vcx is in use and cannot be recompiled.

I have to CLEAR ALL and RELEASE ALL (no need for close all as it closes the project too). Also if I just ignore the error and recomplie second time class is closed and no error happens.

AT