The error is caused because you have too many mod NPC mods, usually the GUI installer(setup-gui.tp2 at least in previously- before the BGTMusic) helps with this thing as it removes the extras...Hey I might have just come accross an error. I'm switching from BG to BGII right now. I uninstalled BGTMusic by hand and then installed the Nalia romance(well actually the one that has her last name in the title, but I forgot how to spell that). When I tried to start up BGII, the game wouldn't start as it was giving some error message. I eventually checked the songlist, and found these entries.
I shortened the list to be only go up to 99 and eveything fixed itself. I checked the post-installer and I didn't see anything to fix this. I suppose it is possible that I accidently edited out the lines of code in the installer that are supposed to fix this, but I still felt I should bring it up.
But understandably that won't help if you don't reinstall the thing... as you should... hmm, the BGTMusic should have option to use the BG2 music, so the mod actually needs to be reinstalled.