Before I had Kapton I just used simple mylar tape. Designed many switching power stages with that and had no problems. I imagine that getting some good tape that will not creep or melt at slightly elevated temperatures would be a priority.

At the moment only components that are getting hot are transformer (secondary winding) and output choke.
I disassembled the atx transformer to see how it is wound. 0.3 mm was used as a conductor. My transformer has the primary winding wound first, then the insulation and then the secondary winding.
Also efficiecy is around 90% (250W primary, full load)

 

1. To heat a coil? Frequency too low. Don't don't just work with the theory, vary the frequency and see what happens with the heating.
2. The magnetic core is not good. There are many types of ferrites and if the frequency is exceeded they heat up.

I like the theory and I would love to talk about it.
Try to measure the inductance of the coil if it is the calculated one.

If the magnetic core does not have the desired properties, it allows the current in the coil to grow faster and heats the wire.
If the magnetic core does not support the frequency in a filter coil it will heat up.

I have an explanation for this phenomenon if you will.
Just as the electrolytic capacitor cannot be used at a very high frequency it will heat up.