1. You use a MassTransit web-client and a proxy server to connect to the Internet.
2. You can download files only from outside your network. There is an error-message «Failed to connect, but connection method did not report a reason for failure» internally.
The MassTransit Web Assistant Plug-in was not designed to work in conjunction with a proxy.
You will need to disable the proxy settings or switch to HTTP mode before attempting to transfer any files. Note that with HTTP mode you cannot transfer files larger than 2 GBs.