What are you setting it up as.. join wireless network .. the very worst setup, it will disappear.. reboot the whole network in order modem. router TC.. clients and it will likely reappear.
Tell us what network setup you are using..
If you setup with cable to a computer completely isolated from the network with TC also isolated.. finish the setup of everything you want. .before update.. then plug it into the network. .then restart everything in correct order.. it will work most of the time.
Thank you for your answer. The problem is that I an not having the chance of doing anything. When I start the setup, the TC disappears from the airport utility program, and after that it gives an error message. I tried using the wireless connection, and also with the ethernet cable. In none it worked.