>Therefore, a rule of thumb: give your subclasses meaningful names, different from parent names.

I made other research and i found the real problem:

the real problem is the names of two subclassed class, the names are the same but the two classes are in different classlibrary.

The scenario is the following :
ClassLibrary CLASSES.VCX contain 'BASEFORM' Class based on VFP base Class FORM
ClassLibrary BASE1.VCX contain 'BASE' Class based Class 'BASEFORM' of CLASSES.VCX
ClassLibrary BASE2.VCX contain 'BASE' Class based Class 'BASEFORM' of CLASSES.VCX
ClassLibrary B1.VCX contain 'SUBBASE1' Class based Class 'BASE' of BASE1.VCX
ClassLibrary B2.VCX contain 'SUBBASE2' Class based Class 'BASE' of BASE2.VCX

Issuing the following commands is possible to reproduce the behavior:

MODI CLASS subbbase1 of b1 (close it) CLASSLIBRARY => base1.vcx
MODI CLASS subbbase2 of b2 (close it) CLASSLIBRARY => base2.vcx
MODI CLASS subbbase1 of b1 CLASSLIBRARY => base2.vcx <=== ERROR should be base1.vcx