I have set up my Urbackup server (2.1.19) on Ubuntu Server 16.04 LTS. I have issues backing up over the internet, but I will post that in a separate topic. This post is purely regarding backing up over LAN.
I typically use UFW on my Ubuntu servers to allow only the incoming ports required, nothing more. You can see from the attached screenshot, I should have all the relevant incoming ports for my Urbackup server to function, but when UFW is enabled clients cannot connect to the Urbackup server at all. With UFW disabled, file and image backups (on LAN) run smoothly.