The plastic hook that hold the rack catches fire..

Any flaws in the painted surface on the inside of the unit will cause a spark and that generates heat. Examine the surfacecor any exposed metal. You have to use special paint to resurface the inside.

Wild guess: Maybe it's making contact with some metalized paper or aluminium foil?

Pry off plastic tab. Clean up burn marks with sand paper and touchup paint. Order a new rack clip. Also clean any burn residue off rack.
MOST IMPORTANT: After the repair, do not put the rack back in unless you are cooking on it. Running the microwave with nothing on the rack is what causes this to happen.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/jeffrey_634abadf963e31e5

Hi there micro
There are things that the microwave does not like. One of them is aluminium or metal for that matter.
You can buy plastic hoods or just cover the plate with plastic film not forgetting to pierce the film in several places in order to let the steam escape.
all the best
Kolbrun

Most rack support problems I have seen are from food splatter or grease getting around the support and even seeping behind it. Once it starts burining it must be replaced. It is really important to keep the area around them clean and dry and free from food splatter. A Microwave cannot tell what is on a plate or on the wall or rack support, and every food particle cooks each time the unit is run, until it turns to a carbon deposit and starts to attract the microwave energy and starts to ARC to that point.
Then it burns up the part it is arcing to. Have to replace support and scrub the area and inside of the cavity perfectly clean.

if you mean the door hook inside the door this is quite easy you need a flat small bladed screwdriver to split apart the plastic inner part of door from the outer keep working around till you have released all the catches there are several, the inner skin will pull away revealing the hook and a spring the hook just slides out refit new hook with spring and reassemble. easy.

Why is this even a question?

Of course a paper towel can get hot enough to catch fire in a microwave. In addition, anything not specifically identified as microwave safe can melt, deform, crack, chip, spark, catch fire, explode, etc...
This is not a microwave problem.

Sonja, I'm not sure what hinges you're speaking about. If you are referring to the bumpers on the ends of the wire racks that are used for convection...move the racks away from the wall. Generally, you should not have anything metal in the unit when it is just microwaving....they are for convection use. You will not start a fire but you will shorten the life of the magnetron if you continue to allow the sparking condition.

If you're talking about a hook-like device that sticks out of the door and goes into a hole in the chassis of the microwave (or any other internal part of the door for that matter) you'll have to take the door apart. Brands and models vary, but some use screws which are obvious, and others use hidden hooks similar to the ones in the drawing I've added to this post. If you can post or e-mail photos of the area where the part goes, we may be able to translate our experience with other brands to yours.