News, I compiled fvwm-2.5.27, restarted, and it works without a hitch. So, it seems that, whatever the problem is (and I am not saying it's in fvwm), it only happens with the current cvs.
I don't know when did this start, I noticed it a couple or three days ago. I am using pure cvs, no extra patches, not even the menu translucency stuff. -- Jesús Guerrero