I had the same problem. Inside the only php file of the plugin there is print_r call. Comment that out and it is fixed. I also commented out one of the next lines that define zero fee. As long as it is 0,00, there is no need (for my usecase) to show on checkout.
Authors, you should really fix that bug, it is shameful to say the least to leave that in. If the functionality is not perfect you should rather disable the option in admin interface.
BR