After removing the winch I first removed the rope and cleaned all the debris from the winch. Then I took the winch apart and tested the motor, which seemed to work out fine. After putting the winch back together again everything worked, most probably the cleaning and refitting the thing solved the problem.
To clean the rope I first loosened the rope threads and then soaked the rope for a night in water and some dishwashing soap. Then I put the rope in the dishwasher and let it run the 50° program, without added soap. I have to say, the result was better than expected!