Empty URLLink.acsm

I'm trying to download an EPUB3 ebook that I purchased. But no matter what I try, the "URLink.acsm" file I get is always empty. I've tried different browsers, even different devices. I've contacted the seller and he sent me a new link, but it's the same problem. Adobe Digital Editions displays the error message "E_ADEPT_NO_TOKEN".

What could be the reason for this problem?