problem in store with product download

I have two sites where I use a store to sell downloadable products (ebook).
For a while ,customers after purchase can't download the products using the link, an error appears. 

I added the product again and replaced the download file.

Unfortunately, nothing works, the problem continues.
Please help me to solve this problem.  

Marta

Did you figure out what the problem was? I am having the same issue. Thanks in advance for your help.