Genmenux does not come with VFP. Mine came with a framework that I use. When I loaded SP3 I had the very same problem you mentioned. This was discussed on the frameworks mail list because others had the same problem. The recommended fix was as shown below. It fixed the problem.

It sounds like you are also using a framework...vpm? It only shows up when you have made a change to your menu and you're generating an app or exe, right?

>>This is not a VFP 6.0 problem. If you'll find your genmenux.prg, search for the following lines and comment them out the problem should go away.
>>
>>*!* IF [2.0]$VERSION()
>>*!* =WARNING("GENMENUX Requires FoxPro 2.5 or greater!")
>>*!* ENDIF
>>
>>Dale
>
>must confess I'm a trifle reluctant to even try it again until or unless I'm forced to do another reinstall - having wasted most of yesterday on that fun and games. But I will make that adjustment to the genmenux program in any case and see what happens next time I risk the sp3 upgrade.
>
>Thanks
>
>Harry