Yes, the mini boss. I was experimenting with some cloth on the character and I should have not. There are some colliders missing and this causes the red arbitrary geometry being generated in the windows release build. It does not seem to happen in the WebGL build.
I disabled the cloth simulation as it was not really making much of a difference anyway and will upload a new version once the upload limitation is lifted. This will fix some other issues as well.
Thank you for playing and your kind and encouraging words.