I think I might know what your problem is. You're accepting file uploads using handleMultipart, yes? Snap kills uploads that are going too slow, otherwise you would be vulnerable to slowloris (http://ha.ckers.org/slowloris/) DoS attacks. What's probably happening here is that you're doing slow work inside the "Iteratee IO a" handler you pass to that function, which makes Snap think the client is trickling bytes to you. If that's the case, either finish the iteratee more quickly and do the slow work back in the Snap handler (preferable), or disable the minimum upload rate guard (although that's not recommended on a server talking to the public internet.)
Ok, so I butchered Snap by replacing all of snap-server's killThread calls with putStrLn calls, and the putStrLn that is triggered by Snap.Internal.Http.Server.SimpleBackend's runSession (line 163 in snap-server 0.8.0.1) seems to be the culprit. Is that a rate limiter, or is that something else? Anyhow, I think there's a bug in there somewhere. I'll be poking at it a bit more, but that seems to be the top-level source of the errors.