printing – the module entity_print -> file_unmanaged_save_data write a pdf with 0 bytes

We have a custom written module (not by me), which uses entity_print to create PDF documents. It was working fine until I updated drush from 9.0-beta4 to 9.5 (I had to update the drush commands). It seems that all functions work the same as before, but the PDF written in the public directory: // is a file with 0 bytes. When I see the / tmp / files created in the process, they have the correct data written in them. Any ideas or advice on how to debug this?