You are not alone. In the thread, however, there’s a link to a possible fix. I’ve not tried it so YMMV, but it seems like a plausible fix.
Given that Mongrel is no longer being updated, though, why not switch to something like Thin?
Edit: Sorry, I see that you're asking about 1.9.3. My understanding is that Mongrel is (supposed to be) compatible with up to 1.9.1, but broken in 1.9.2+. Yet another good reason to switch to something more modern and maintained.