proxy servers are always worth mentioning.
If this is what is happening in your case, I patched this issue after version 220.127.116.11 was released and I uploaded the patch to our codeplex download page for that version. For the patched solution to work depends on if the proxy server is providing any custom server variable that can be used to help the web server know when it is handling a secure request (since it is not an https request from the web server point of view). See the article using an SSL Proxy Server for more information about how to configure detection of the server variables, but keep in mind that would only work if you upgrade to 18.104.22.168 and then apply the patched version of mojoPortal.Web.dll
however if this proxy is changing the the hostname/domain name and the web server is not seeing the real host/domain name (ie the host/domain that the users sees in the browser) then the service url may still be wrong so it may not work for you, I'm just not sure.
another option is you could put this in user.config or Web.config
<add key="ForceLegacyFileUpload" value="true" />