Why does this error not occur when I'm starting the BGT-Setup.exe directly?
Well you are using a .bat from 4 years ago... I have to rhetorically ask: Is there even a point in figuring it out ? ... cause for me, it wouldn't matter if it's the method or insert or a mistake in the .bat file, which I would remember there to have been a few times. For example the .bat not reading the insert it was provided and instead of using the default path forcibly*, which was: C:\Program Files\Black Isle\Baldur's Gate ... which was fine most of the time, until the Windows made it useless.
Back to the point, using the BWS instead, you'll be assured to avoid this and the tons of other things that have been fixed for years. And the install should be easier, as you don't need to do any of the heavy lifting as the setup will do all the downloading, extraction and installing and other manual things you need to do, except selecting mods and their components. Or you can select the auto selections, but I recommend that you do NOT use any of them. Cause you have the option to not. Using the "..." button next to the compilation selection.
*this was because the BGT-weidu itself altered the insert method a few times.
Edited by The Imp, 22 October 2015 - 02:54 AM.
Yep, Jarno Mikkola. my Mega Mod FAQ. Use of the BWS, and how to use it(scroll down that post a bit).
OK, desert dweller, welcome to the sanity, you are free to search for the limit, it's out there, we drew it in the sand. Ouh, actually it was still snow then.. but anyways.