We're currently debating whether or not Ariadne's header methods ( ldHeader, ldSetContent, ar('loader')->content(), etc. ) should fail silently. This is something left over from the early Ariadne days when this still seemed like a good idea... but to change it now may break quite a lot of ariadne sites.
Hope this helps,
Auke van Slooten
PS. when I access the above URL it sends the content-type as text/css, so it does seem like what I described is the problem here.