The Backup Clients communicate with the Backup Server over TCP/IP and the Backup Server receives their backup data using Port 2125. To see if this port is open and allows for TCP/IP inbound traffic, press the Test Access through Firewall button now. If a firewall is blocking Port 2125, your computers on your network will not be able to communicate with the Backup Server.
Firewall configuration issues are a common culprit that can prevent the Backup Client from logging into the Backup Server. Firewalls tend to block TCP/IP communications and can cause a problem preventing proper communication between the Backup Client and Backup Server. When the firewall test fails, you need to modify the settings of your firewall on this computer and change the settings to allow for inbound TCP on Port 2125 from any IP or Port.