Are you sure you have created the "eng" output dir?yeah that was a typo on my end. My exact thing being typed out is "python convert_quest.py C:\jpn\q1010003.arc C:\jpn\eng\q1010003.arc"
The error message is "The filename, directory name, or volume lable syntax is incorrect."
Maybe try this batch script I used, it works fine for me. Just place it in the folder where all your quests are.
mkdir Converted
for %%f IN (*.arc) DO convert_quest.py %%f Converted\%%f
pause