undefined and download error

Hi,

For few weeks now, I have a problem with delightful downloads. When I create a new download or if I click on old download, I have a popup windows with bystudio.fr indique:
undefined
I don’t know where it is coming from.
The main problem is that sometimes, when a big file is downloaded through delightful downloads, the file is corrupted i.e, if it is a zip file, it is impossible to unzip it. If I do a direct download, the file do not have any problem.

Do you think those two problems are linked ? How to solve that ?

Thank you

delightful downloads version: 1.6.6
Server: Apache
PHP Version: 7.0.15
MySQL Version: 5.6.34

PHP Safe Mode: No
PHP Memory Limit: 512M
PHP Time Limit: 300
PHP Max Post Size: 64M
PHP Max Upload Size: 128M

## WordPress Information ##

WordPress Version: 4.8
Multisite: No
Max Upload Size: 250.0 MB

Site Address: https://bystudio.fr
WordPress Address: https://bystudio.fr