Thank you for your quick response!
I used the files you sent me, and I made a simple questionnaire file (containing these RT tasks while keeping txt files in the stim subfolder). I also changed the file names so there is no underscore now.
However, when I get to the 2nd RT task (food-affective.csv), there is still an item saying "$pos.txt not found".....I have zipped the whole questionniare and uploaded it here. Would you mind going through the questionnarie and see if it happens on your computer, too?