how large a file were you attempting to post?
edit: nm. just found it. there's an issue with the ssl renegotiation and file posting. we'd thought we'd nailed that with a config change, but it looks like we missed something.
you can help us debug this by trying a post on a fresh copy of your browser (i know, i know) as its related to file posts during an ssl renegotiation handshake.
thanks, sorry :-\