First back up and understand, ssl certificates work in the transport layer not the application layer. mojoPortal code does not use the certificate or interact with it. mojoPortal code runs on the web server and the certificate is not even installed on the web server but on another server.
your certificate is working and mojoPortal is not preventing it, the only issue is mojoPortal cannot do the redirecting from insecure to secure requests in your setup as it can when the certificate is installed on the web server. So you must use other external means to handle that.
mojoPortal does log all unhandled errors, so nothing in the log tells me the file upload is not even reaching mojoPortal code but is blocked by the proxy or the htaccess in some way before ever reaching mojoPortal. I think you should press that with the host.
Finally, I have spent a lot of my time trying to help you and I am not your employee and my time is valuable and I am certainly out at least $150 of value on my end too. I did not sell you your ssl certificate nor the use of the proxy server that came with it.
I tried my best to help but there was never a guarantee that mojoPortal could do the redirecting with this proxy server ssl setup, and it is not my fault nor fault in mojoPortal code.