There's an easy way to look up the final URL you get redirected to: just point your web browser at your main Sync Server URL at
http://sync.omnigroup.com/USERNAME, and it will be redirected to the appropriate server at
https://syncNN.omnigroup.com/USERNAME/. You can grab your final URL from the location bar and use it in Transmit, Finder, or any other tool that has trouble handling web redirects.
(WebDAV is the web's standard http protocol, with some extra extensions to cover other common file operations like renaming and locking files.)