I looked in the code and found the error.
On line 1038 and line 1080 there is the copy function. There was added an esc_url() but unfortunately the ) was on the wrong place.
This is what it is (on line 1038):
copy(esc_url($createDirectory.'/'.$nameOfPdf.'-'.sanitize_text_field($_SESSION['pdf_uniqueid']).'.pdf', $createDirectory.'/'.$nameOfPdf.'.pdf'));
This is what it should be:
copy(esc_url($createDirectory.'/'.$nameOfPdf.'-'.sanitize_text_field($_SESSION['pdf_uniqueid']).'.pdf'), $createDirectory.'/'.$nameOfPdf.'.pdf');
I made an fix for it and testet it, it’s working like a charm.
You can download it here: https://cloud.lus.one/s/Gwi9fCkNwSnid66