The file name is correctly spelled, no upper case letters. I also tried putting the file in different subdirectories, including the one of the theme but it didn't change anything.
I'm quite sure that polylang reads the file because at one point it gave me an error message in the settings page (when I added some taxonomy strings for translation that were already covered in the settings).
Anyways, I think it has something to do with the new Pytheas code and maybe with the fact that the options framework is now integrated in the theme (no need to install the plugin). For now I rolled back to the old version of the theme and the wpml-config works without problems.