DIAGNOSIS The hot air isn't circulating properly. This is almost always because the square crisper plate inside the basket is clogged with grease or food bits, or you're overfilling the basket.
FIX 1) Unplug and let cool. Remove the crisper plate from the basket. Soak it in hot, soapy water, then scrub it clean, paying attention to all the holes. A stiff brush or even a toothpick helps. 2) When cooking, don't pile food. Use a single layer with space between items. Shake or turn food halfway through the cook time—this is crucial for even air flow. 3) For foods that release a lot of grease (like fatty meats), place a piece of foil or a small oven-safe dish on the crisper plate to catch drips.
IF THAT DIDN'T WORK The fan might be obstructed. Check the top of the cooking chamber near the heating element for a major buildup of grease or debris that could be blocking airflow. Clean it gently with a brush as described in the cleaning question.
HEADS UP Lining the entire basket with parchment or foil blocks the holes in the crisper plate and will cause exactly this burning problem. If you use a liner, it must be perforated or only cover part of the plate.