Well, for the past couple days, I've found I've been periodically blocked from the server on grounds of "CD Key in use". After a bit of googling, I found this is because the GoG version of the game uses generic CD keys. This means, under normal circumstances, only one GoG player may play on the server at a time.
So, if anyone else is having this problem, visit this page:
http://www.gog.com/support/neverwinter_nights_diamond_edition/online_play_in_neverwinter_nights_diamond
And request a unique CD key. It'll fix the problem cleanly.
edit: Looks like theres already a thread for this- for some reason just doesn't turn up in forum search:
http://www.escapefromundeath.com/forums/showthread.php?t=74437