Ok, i think i know where the problem is, but i don't know how to fix it.
I never buy a product without testing it first. So I installed a version from a colleague. It worked fine. After that I purchased my own copy.
But, the copy from my friend installed in a folder called 'eStore' and the copy I bought a couple of days ago installed in a folder called 'WP cart for digital products'.
The folder eSotre no longer exists, but when I create an encrypted download link for a product this folder is still referenced.
Any idea on how i can fix this?