It looks like the problem is actually,
(wrong-type-argument number-or-marker-p ‹\213)
And what shows up as the first line of the buffer is:
^_\213^H^@^@^@^@^@^@^C^C^@^@^@^@^@^@^@^@^@HTTP/1.1 200 OK^M
So it would seem that there is garbage getting inserted in the http buffer. Is it possible that buffers are being reused and not properly cleaned before the next request? Will have to do some more digging...