Hmmm. It’s strange. I don’t know.
Does your urbackup server behind NAT? If yes, may by you incorrectly port mapped 55415/tcp to server?
Or firewall problems.
For understand the problem at the server side or client side try to install install pre-configured client to windows at the same network as your current client. Will it work or not? If that client won’t work the problem ins’t at the client side.