Are you connecting to
http://sync.omnigroup.com (incorrect) or
http://sync.omnigroup.com/username/ (correct)? If the latter, and it still isn't working, try replacing sync with sync1, sync2, or sync3. Omni rejiggered the sync server to keep up with the load and some clients (like the Finder in Snow Leopard, for example) don't follow the redirect properly. Maybe Transmit is one of them.