Anonymous issue posting broken (BB-8916)

Using Chrome Version 28.0.1500.71 m, attempting to post an anonymous issue on a public issue tracker that allows anonymous posting:

Input error: Invalid referer
This field is required.

(despite all required fields being filled in)

This is under a custom domain, and the repository itself is private. I suspect this is a recent problem, since I recall being able to post such issues anonymously before.