Thanks for reporting this! I bet this happened because of the HN redesign. There was some other stuff that broke on the site when that happened: http://arclanguage.org/item?id=18744 . I'll email the people at YC that can fix it.
Until they get it fixed, you can try Anarki instead. Anarki is a fork of Arc that's been changed somewhat, both to fix bugs and to add features. https://github.com/arclanguage/anarki .