I experience the same problem on a regular basis and I have not found a solution yet. The drive between the propulsion pulley and the stub shaft appears to be via a M12 Nyloc nut and an aluminium drive spider attached to the pulley. The system appears to be a simple torque limiting device where if you drive the mower too hard the aluminium spider slips under the washer axially compressed by the Nyloc nut The nut then loses drive and the drive via the stub shaft fails to turn the propulsion pulley It has happened appoximately 7 times so frequent that I have made a jig out of Dexion to lock on the roller so I can really tighten the nut whilst the roller is held stationary. It still fails and I have just tried tightening the nut as tight as possible and applying Loctite 243 nut lock to stop the nut slackening off.
Rod Billson, September 2013