I think this is actually be caused by https://github.com/xwiki/xwiki-platform/commit/bf2c7f3c983ef5b8bb06f9619619116fb39a71db which has been cherry-picked to stable-14.10.x: https://github.com/xwiki/xwiki-platform/commit/bf2c7f3c983ef5b8bb06f9619619116fb39a71db which changed the type from CSS to LESS.