Can't use 7777 port as a file transfer proxy

I’ve tried to disable File transfer proxy in Admin Console temporary for testing. It was 7777 port before that. But i can’t make it get enabled with the same port again. It always jump back to disabled state after i press Save. If i use some other value (7778, 7775) it saves correctly. I don’t see this port being used in System properties or other menus. Wonder where the conflict is?

2014.01.26 12:57:00 org.jivesoftware.openfire.filetransfer.proxy.ProxyConnectionManager - Error creating server socket
java.net.BindException: Address already in use: JVM_Bind
at java.net.DualStackPlainSocketImpl.bind0(Native Method)
at java.net.DualStackPlainSocketImpl.socketBind(Unknown Source)
at java.net.AbstractPlainSocketImpl.bind(Unknown Source)
at java.net.PlainSocketImpl.bind(Unknown Source)
at java.net.ServerSocket.bind(Unknown Source)