Hello,
I've installed the game on Linux using the original Bioware client + patches and the data extracted from the Diamond Edition GOG installer. I know there's an installation process on ./play.it but I prefer to manually install the game and I'm not sure it covers my issue anyway.

All works fine except for the three premium modules included. When I try to start them, I get an authentication dialog that seems to be stuck (from what I understand, the Bioware servers have been shutdown a while ago). I've seen elsewhere that premium modules for NWN are authenticated by .dat files placed in a "premium" subdir in the game's dir. So I digged into the data files and saw this dir that I didn't copy at first. There's also one with the KingMaker installer available on my GOG account with different (bigger) .dat fles. But whatever version of the files I copy, when I try to start a premium module in NWN, I immediately get an authentication error (#110623).

I'm not sur if the issue comes from my installation or is more generic: are the premium modules still supposed to work (the Bioware website has been completely updated by EA and I think everything related to NWN has been dropped) or is it an issue with my setup? If so, any idea how I can authenticate the modules properly?

If you are talking about the premium modules not included in the GOG release:

You might want to check this thread and websites mentioned in there:

https://www.gog.com/forum/neverwinter_nights_series/nwn_premium_modules

Yes :D

Check in your account here on gog, view downloads -> more ->serial keys

And what you described is the way to go, also here mentioned prior on the forum :D I think went to page 2....This question comes up once over a while....

EDIT....I think they included this finally as well in the solution on their support page (how too) but not 100% sure.

But in my case, that's the opposite: if I use the serial keys from my GOG account, the premium modules don't work; if I use the keys included in the installer files, they work...

Let me try to find it....
I did had to do some fiddling myself before GOG (and others) released an exe to do this for you....

EDIT:

Had to go through a few threads :D
https://www.gog.com/forum/neverwinter_nights_series/how_to_install_nwn2/post3
I thinkit works the same as the fist NWN. Just go into the main folder where you installed NWN2 and look for a file named nwncdkey.ini, open it with Notepad (or any other text editor) and enter your keys into that file (just overwrite whatever is already in there). You should be good to go after that. Of course, just in case, make a backup of the file before altering it.

EDIT2: make sure to use an editor, nothing like openoffice or similar....it needs to be pure editor, if there are other characters added it will not work.