Hey Prokop,
I had that problem with a Solo 640. What happens is that the oil pump drive shaft with the worm gear gets very hot from proximity with the clutch housing when the saw has been run hard and is then shut off so that the pump shaft is stationary near the clutch drum. The shaft then warps ever so slightly so that the gear mesh between the drive gear on the crankshaft and the worm is too large, and the gears jam or strip.
I called and talked to Solo USA tech support. They were AWESOME in explaining the situation to me, and they shipped me a complete replacement oil pump assembly including a re-designed driveshaft, no charge. It's not a joy to install (have to remove clutch), but I've had no subsequent problems.
- Jay