Please double check that you've used the troubleshooting section of the product manual in order to overcome the issue. You might try choosing other modes of the device before choosing oven or microwave to see if the issue can be side-stepped. But then again, if that method works, then you might still need to get the unit checked by a authorized technician. Thank you.
How new is it, installed today? It takes 24 hours to settle.Also, if it was laid down during delivery, it should be left standing upright for the refrigerant gas and oil to settle before turning the compressor on.
The frost builds up during the day and a timer starts a defrost cycle, once, sometimes twice a day. This normally happens during the night, so you wake up to a wet fridge. The melted water runs into the defrost drain which often have a small P-Trap at the rear. Yours is blocked. If the frost you see is completely blocking the evaporator fins, either the defrost timer or the defrost heater has failed.