Are you sure the server isn't full? If the server is full, you can't log in of course. (Though keep trying, you'll log in eventually! Trust me, I do it all the time.)
If it isn't that, then it's offline for work / updates, so just be patient if that's the course. And finally, if that's not the problem, then re-download the client. I've had to do that a couple of times along with some others. So maybe try that.
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Hope I was able to help!
As he said he have Membership so he should be able to go in game when the server is full. So there is maybe a little issue with your Membership.
Well, maybe I'm wrong so you should wait for a answer from a Staff member.