Are you using certificates? If so you can reset them from the Console Tools CTRL + ALT + T then select "Time Configuration" just take the defaults to the end. If that does not work then reset the network settings again in Console by entering CTRL + ALT + I then go back and change your IP Address, Subnet, Gateway and DNS (If need be) by entering CTRL + ALT + N. Does this happen with other systems from the network as well?