I got to a point in the campaign where I couldn't progress due to this error. I deleted the game and reinstalled it and then tried starting a new file with a new character, but got the error yet again in a completely different, earlier part of the game. I deleted and reinstalled it yet again and am about to start over once more. Any hints on not having to do it a fourth time? (For anyone about to say it, I haven't been utilizing the toolset whatsoever, so that's not the issue.)

Where did you install the game?
Did you disable UAC & your antivirus software while you installed it?

I installed it on an external hard drive; E:\GOG Games\Neverwinter Nights 2 Complete. I always have UAC disabled because it's pointless, and my anti-virus wasn't giving any warnings about NWN during installation, so I didn't bother doing anything special with that. I'm having trouble with the game deleting files as I play, not my computer deleting files during install.

Does your Documents\Neverwinter Nights 2 folder show your user as the owner, with permission for full control?
Do you have enough free space on your C:\ drive?

Yes to both.

Do you have any errors in event viewer that correspond to installing or playing NWN2?

None whatsoever.

Supposedly you can backup the old saves, remove and reinstall the game and then continue with the old saves. Might fix the thing and might not, worth a try maybe.

Also, just a hunch but you might try to install to C if there's room.
Could be something gets gunked up with NWN2 working with the external, even if there' s no reason why that should be.

That first thing doesn't work, due to the way NWN2 creates its saves and the nature of this specific error. I thought about the C:/ thing, but unfortunately I don't have any room on it, since it's a fairly small SSD (hence the need for an external).

Check where your Temp folder is (Control Panel/System/Advanced System Settings/Advanced/Environment Variables). NWN2 extracts module data to Temp so if it is on C: and you lack space (NWN2 writes 50-100MB to its Temp folder IIRC) then that could cause the error.

If this is the case, you can change the Temp folder to a drive with more space.