This seems to be a bug, I just ran into it too. The webui plugin does not honour the -b option and so tries to upload torrents to /upload, when (in my setup) it should be /deluge/upload. I had to hack the source code to insert the /deluge/ prefix to fix this, but this did indeed fix it.
As another note, I also had to manually edit the source to prevent the webui from listening on all interfaces, as once proxied, I only wanted it to listen on localhost. This is not supported either and I had to hard code it to make it work for now.