Since the beginning of the betas I keep getting every now and then this annoying error. It occurs only when I'm logging on the beta client. With the live client I never got such an error, hell I never got any error ever. I tried to Google it a bit but so far I couldn't find a solution for this beside something with the DNS server and stuff like that which I don't really wanna bother with. I also noticed that most of the reports for this error are from 2011. I don't know if meanwhile it's been found a fix for it, reason why I don't get it on live, but on the beta "it came back", so to say.

P.S. The full name of the error is
"The login request was no valid. Please try again later (Error #2016)"