How to fix my DEM 10 dehumidifier?

I've had this DEM 10 for three years with perfect operation, until two days ago.

1 The unit runs for 4 seconds then frost light indicator comes on and machine stops running. Left unplugged for 24 hours.
Plug in turn on DEM 10 runs 4 secs, defrost indicator comes on and machine stops running. What is likely cause?

The air filter was quite clean, but I cleaned it anyway. Still same problem

2 Which screws to remove cover?