Ok, I found the problem. Disable the Feature "CSS bundling and minification" in "General and miscellaneous Settings", "SEO Settings". It's added to the to-do-list, to check why this theme is not compatible with this feature.
Best regards, Peter Software Developer www.getyournet.ch
It is working when I disable the settings. The strange thing the problem is not a constant one. I have built a new website with the same build of nopCommerce(it is customized by me) and it is working on the same server I sent you yesterday.
Difficult to analyse, because I don't know how your Serial Number is implemented (Plugin ?). This theme overrides the View ../Customer/DownloadableProducts.cshtml Rename this View in the Theme Folder and see if something changes.
Best regards, Peter Software Developer www.getyournet.ch
Site prepared by me finally went online :) It's mostly in Polish language right now (English in work) and I had to make many modifications and update to 3.6 but it's still Theme X :)