The problem is with the chunking that apache does to avoid having
to read the entire response, so it can set the Content-Length header
properly. I have no idea who apache does chunking, but on the 9.0
client it was getting confused and trash characters were leaking into
the data. The trash is probably the result of the client getting out
of sync. 

Anyway, I do not know who is at fault (client code or our ancient
apache server), but after many hours of screwing around I found out
how to prevent the client from getting chunked data from the server.

Basically, you have to force the request to be sent as HTTP/1.0
instead of 1.1, which tells apache to not chunk the data. In general
this is not a good thing to do, but in this case the RPCs are pretty
small and not on a critical performance path. We can revisit when we
upgrade our boss to apache 2.X ...