You’‘ll want to make sure that you have port 7777 open on the server, that is the file transfer proxy. Second, you’'ll want to make sure we setup the correct server address. How do people access your server over jabber, is there a DNS entry?
I wonder if this is a client problem, as you are using Spark you may launch it two times on your local PC with two accounts and test if filetransfer works in this really not useful setup.