So, I'm having this same problem. My file structure seems correct (although, my folder seems to be nwm instead of nwn). I Have downloaded the full pack twice and I still get the error. The TLK file i have is 8289k, is that the right size?
It's interesting that the other poster was asking about this only a couple weeks ago, I bet we are both using the GOG version. I bought the GoG edition and had to do change my CD keys to enable online play and I have not yet re-installed the "premium modules". I'm wondering if maybe that would prevent the Ravenloft modules from working too. I don't expect that to be the solution but I will try it anyway.
Has anybody figured out what's going on? Does anybody else know how to fix or attack this problem? I don't think it's my file structure.