Podcast Artwork not working despite correct size, file format and color space

Hi!

Like a lot of people previously in this forum, i cannot get my artwork to work with apple podcasts. The URL to the .xml file is: http://kradient.ch/PhiloText/rss.xml. The image: http://kradient.ch/PhiloText/logo.png.

The image is 3000x3000 pixels and in the sRGB color space. The server should allow HTTP head requests. And yet I get an aforementioned response on the podcastsconnect website (in german). I wrote the xml file myself, if that is of any relevance.

Can anyone help?