status 500 on Live Edit and SmartEdit

Trying to open some pages (such as Product Details or Product List) on Live Edit or SmartEdit it redirects to an error page (HTTP Status page 500) and the response obtained is a status code 500. While on other pages, such as Home page, the page is displayed correctly.

I read on the guide for SmartEdit that page retrieval is done through a request "/previewServlet", but I could not find where this request is being mapped. With the debugging I noticed that the request returns status code 500 on the pages where I get the error and status code 302 on the pages that are displayed correctly. Where and how is the request "/previewServlet" managed? Thanks