[Arco Witter]

Thanks for answering and the idea of the arguments!!

Unfortunately the files are still dumped in the wrong folder, currently the folder has this name:
program_ folderondevice(field('title'), field('#genre'),field('series'), field('series_index'), field('#max_number_in_series'), 10)
File names are:
de wetten van de magie 00 proloog - de aflossing.jpg
de wetten van de magie 00 proloog - de aflossing.mobi
etc.

It's just weird that this does work with most books. I did not see any difference in the meta information of the books.
However, currently it seems that all books are dumped in this wrong folder, so using 'program:' for the columns does not work.

edit: It looks like the export script is evaluated to remove all characters that are illegal in files before it is evaluated for script/functions.
However, when I use a '/' in the genre, I see it replaced with a '_' on exporting. Still the value of the column is ok.
Is it possible to use the column value without it evaluating completely when exporting? (Since it has already been evaluated when Calibre started or meta information changed)

edit2:
I replaced everything as it was: function is still the same. Changed the column back to: {title:folderOnDevice({#genre},{series},{series_in dex},{#max_number_in_series},10)}
also changed the export script to: {#folder_on_device_10:||/}{title}

Now it works for each book!!