"Authentication failed" when attempting to upgrade to 6.0

Hi. I attempted to upgrade to 6.0 using the Update Manager but I encountered this problem:

It starts the "Downloading the release upgrade too" process, but a few seconds in it gives an error message. The error message is titled "Authentication failed" and the full text is "Authenticating the upgrade failed. There may be a problem with the network or with or with the server."

Does anyone know what causes this problem. Or more importantly, how to fix it? Any help at all will be greatly appreciated.