The following error usually means you are running out of RAM memory on your server:
Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 18353907 bytes) in 
Hosting4445488htmlwp-contentpluginswp-cart-for-digital-productsdownload.php on line 114
This can happen if you are hosting on a shared hosting and you are using more than the RAM allocated to you.
When this happens the server randomly kills any running script to free up memory. This would mean that when you start the download process the script is getting killed by the server so it’s not completing.
Try unchecking the “Downloadable” option on this product. This will make the browser handle the download rather than the script.