Hey guys, so I sent a ticket and received a CD-Key (very quick service btw) and replaced the old file with the new file.
After that nothing changed, I was able to log onto the character screen for the server (Arelith if it matters), made a character and got the same old authentication denied message.

Then I tried reinstalling the game (this time apparently not in GOG Galaxy so now I've got two games installed), copyed the keys into the file instead of replacing and the exact same thing happened.

Did I do something wrong? Does Arelith not work with these keys? I've already googled and didn't find anything that would solve the problem. Anyone able to help me?

You probably got the same mail I did but still I wanted to remind you.

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Please extract the attachment and replace the nwncdkey.ini file in your Neverwinter Nights Diamond game folder (C:\GOG Games\Neverwinter Nights Diamond Edition\ by default).

Next, please go to your My Account page and find the "premium modules re-installer", under the Neverwinter Nights additional content. Download this file and install it. The modules will now be adapted to the new CD keys.
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So what you need to do is download the module re-installer and run it.

I did do that, though not for the one outside Galaxy, now that I'm re-reading the email though I'm noticing that I was supposed to replace it anyway..