First, make sure that the internet connection is working fine. The network connection can be verified in login screen > Settings > Set Network Test > Ping www.example.com. The network connection can also be verified in B5M main view in the bottom left corner - the pop-up from the network symbol should show 'Network is OK'.
Then, make sure that you are able to connect to Apple servers. Make sure that the firewall is not blocking Apple servers (see firewall settings and hardware requirements from the user manual). The network connection to Apple servers be verified in login screen > Settings > Set Network Test > Ping itunes.apple.com. A better way to verify the connection to Apple servers is to test the same network connection with a normal Windows/Linux PC: try to open 'http://itunes.apple.com/check/version' in the web browser - it should return XML file from Apple.
Also, check that you have enough free space on your hard drive, or if you are not using permanent installation for Blancco 5 Mobile, make sure you have enough RAM for storing the firmware images.