Trying to launch GUI apps gives errors with libpixbufloader-svg.so. It fails to find this file. Indeed, for some reason it is called libpixbufloader_svg.so instead of libpixbufloader-svg.so. I copied it to libpixbufloader-svg.so and now LightDM starts and everything else works as expected…
Like, wow! Just one character in a file name, which for some reason got changed, and my whole machine got unusable…
edit: The better solution is to leave the .so files the way they are and just run gdk-pixbuf-query-loaders --update-cache.