"Failed to validate Galactic Civilizations registration information. Please verify that you have entered your email adress and serial number correctly."
I did that several times now, even tried to re-register, but the message keeps repeating itself.
Try this: Allow SDC to update. Right mouse click on "GalCiv Forum". Select "Reset List". SDC will download a new list of forums. After doing that GalCiv Forum worked for me.