Return Bug ?

Do you mean that after writing such a program:

Function test$()
	file$ = "test line"
	Return file$
End Function

Print test$()
WaitKey()

End

You receive a MAV?

If that's it, either with or without FastExt, it isn't the case here. There must be something else too.

Could you please be more specific?

Since you mentioned FastExt, even though you say that the problem is there only with the latest B3D update, are you sure you put the DLL in the apps dir and the decls file in B3D's userlibs folder?