What else can I check?

Clearly the motor and the controller are not fine - from what you say the motor appears to work off-load (removed and tested) but not when installed in the machine = motor not OK.

Your description of the fault condition is not at all clear. You suggest - but do not say - that when back in the machine the motor will "try to work" - what actually happens? Does it rotate at all?

I would discount a fault with the controller and concentrate on the motor - does it have a start-run capacitor? Have you checked (examined) the condition of the commutator and brushes? The sparking under load is a big clue.

Presumably the motor drives a running-track endless belt. Is it water damaged or binding on its bearings and creating excessive load for the motor to cope with?

The blinking of the unspecified light may or may not be significant. The word "leer" confuses slightly.

Good luck...

Peccavi
May 2013