All the files I upload via Download Monitor are corrupted. The appropriate shortcode is in the post and it worked well before. The graphic button shows, no error is displayed on the front end when downloading, but if you try to open the file, it is corrupted.
This does not happen with files uploaded via straight FTP (like FileZilla or file manager in cPanel), so therefore it is related to Download Monitor in some way. In addition, this only happens when "Force download" box is checked, when it is not, the file (pdf) opens in the browser window just fine.