Hi Lassala,
I don't know AddProp5.fll so I may be totally wrong in this. Anyway here it goes...
have you tried to create an out-of-process component that uses this library? I am saying this because in-process COM DLLs cannot have user interface actions. If this Library is issuing such actions (even a simple WAIT WINDOW) your code will crash.
I think that a simple compilation in an out-of-process component would answer if this is a COM subsystem issue or a VFP DLLs one.
HIH