I was having this same issue - I would enter my GW game account details at the NCSoft site when trying to link my game account to my master account, and it kept saying invalid account.
Support suggested clearing my browser cache which didn't help. I figured out that the NCSoft site didn't like my password because it contained special characters that the site doesn't allow in passwords (.*!). Once I changed my GW game account password to be alphanumeric only, the NCSoft site linked my account right away.
I told Support about the issue, hopefully they add it to the Help section of the NCSoft site.