How old is the pump? Is the propeller unit the type with a white shaft and base or a black shaft and base? My guess is the base wore to the point that the drive unit is no longer properly aligned in the magnetic field, that is what the disks at the base of the shaft do, they provide a critical spacing. Send it in for repair, I suspect you have the older white base drive unit with the stainless steel and white disk, likely the stainless steel one was lost at some point and the plastic parts wore.
Tunze USA
305 Victor St
Austin TX 78753