I've restored a backup of some days ago.
It is the same backup from which I've recovered folders during my tests... well, now WHDLoad works.
I've tried to recover folders from my latest WB (the one that doesn't work). I've copied C, S, Prefs, WBStartup and WHDLoad still works!
So... now it's ok, I'd have discovered what was the matter but I had no success.
I'm going to backup again my sys partition...