Thanks for the log.
Your plugboard is missing the last line. It must be
Code:
program:
comma = '';
res = '';
for author in $authors separator '&':
res = strcat(res, comma, swap_around_comma(author));
comma = ', '
rof;
res
Without the "res" at the end the plugboard always returns a comma. I suspect the Kobo thinks a comma is two empty author names.
Assuming the books are on the device, you shouldn't need to send a book to test it. Simply connect, let the process finish, then disconnect, and the metadata will be updated.
Also, remember that after you send a book, the book's series will be set the second time you connect the device.