Cullen12: please explain to me how I can import selective members of my party in order to try this test you describe. I did not at all realize that that might be an option. I mean, the option was not presented to me when I went to import my already exported party to HoW.
When you create the game and are on the party selection/creation screen, when you get to the screen where you select the gender, look down to the bottom right of that screen. There's an Import button. Click on that. A list of possible characters will pop up. In that list will be IWD1, IWD2, IWD3, IWD4, IWD5, and IWD6. These are your characters from the original game (they're automatically exported just before the final boss in IWD). Choose one of them and the character will be brought up**. You then need to choose the appearance and voice set (appearance is defaulted to whatever you chose in the first game, but you can opt to change it if you want. Voice set defaults to the first in the list. Choose whatever you like). Then you have to choose the name of the character (default is whatever you named it in the first game). Once you're done that, click on Accept in the bottom right (under the Import button). You'll be taken to the party selections screen. Hit done and you're good to go. You'll have to repeat this process for each character.
For this process of elimination, that means shutting down and restarting several times, but hopefully this will isolate the issue. If that still doesn't work, then perhaps try importing one of the pre-made high level characters. They should be the first ones in the list (I believe they have a + sign at the end of the name).. If that still doesn't work, then I'm stumped. But try that first and post back if you're still having issues.
*edit* Oh, when choosing the expansion, if you already have high level characters available, the game will say that it notices you have them and would you like to use them. Choose No. This will then allow you to go through the process I described above.
**Of note: any containers your character is carrying are not exported. So scroll cases, bags of holding, gem bags, are not exported with the character. Anything in them will be lost as well.