That says your filenames are too long and calibre is truncating to keep the total path/filename length below ~185 characters.
My solution to this issues was to truncate the author sort and authors to use only the first entry in the list when generating directory and filenames. For your BOOKS directory in the root, this would have you changing your template for send to device to:
Code:
BOOKS/{author_sort:sublist(0,1,&)}/{title} - {authors:sublist(0,1,&)}
A horrible example of a book that will break the maximum length without using that truncation is a book that my spouse read:
Frog Challenge Anthology [02] - 1001 Dark Nights Short Story Anthology 2020 - Fiona Archer & Louisa Bacio & Laura M. Baird & Kait Ballenger & Asa Maria Bradley & Eden Bradley & Boone Brux & C. G. Burnette & Cara Carnes & Caraway Carter & Sierra Cartwright & S. A. Clayton & Katana Collins & Janie Crouch & Patricia D. Eddy & Zoey Ellis & Zoe Hill & Sara L. Hudson & Tracey Jerald & A. D. Justice & Terri E. Laine & Michelle Mars & Gina L. Maxwell & Kayt Miller & Dani René & Gen Ryan & Blue Saffire & Ella Sheridan & Mayra Statham & Delancey Stewart & Jen Talty & Mary Ting & Reina Torres & Magan Vernon & Jennifer Woodhull & Khloe Wren
The filename that I send to her Libra 2 would be:
Elibrary/Archer, Fiona/1001 Dark Nights Short Story Anthology 2020 - Fiona Archer
The Series/series index is not part of the filename when sent to device since a Kobo ereader has the ability to display series information.