If this rings a bell, it's because it has been around for a while and has
been reported before. I can't find it now though. I think it was fixed in
RH5.2 but reappeared in RH6.0.

Anyway, the problem is that the Popup invoked by the start button has a
bogus window id as it's context, so when you invoke a window operation from
the menu, it kills fvwm2. :-( I have made a patch (and a new rpm spec and
rpms) which fix this. You can get them from the above URL (item 3).

The same bug is present in the latest fvwm2-2.3 snapshot, so I'll report it
to the fvwm-workers.

While hacking, I also fixed the FvwmTaskBarMailBox option and the failure
of the start button to pop out again once pressed -- same patch.