Are you sure everything is wired correctly?
I wouldn't expect the resistor to fail like that, as all it does is control how much current goes to the motor. All I'd think the wrong resistor would do, would be either not let enough current reach the motor, or too much current.
I'd try contacting KB and see what they suggest.

What voltage are you powering the KBLC with?
For a 90V motor, it needs to be powered with 115Vac...

Also, on a side note, where did you get your KB drive?
(I'm currently looking for a DC motor controller, without the expense of shipping one from across the pond)