If you are importing PayPal transactions (from your Zen Cart!) to Quickbooks using PayPal to QB Link from Big Red Consulting, you need the new version of this software to deal with Paypal’s new download format! PayPal finally did away with their legacy downloader (and its associated format), and now all users must use the new download interface. This is a new change; as recently as Oct 1, 2018, I was able to use the Legacy downloader.
Grab the updated PayPal to QB Link (sometimes just called PayPal Link) here: http://bigredconsulting.com/latest-version-info/
You will need to re-import your QB lists and modify the download format within PayPal, but the documentation will lead you through all of this. BIG UPS to Big Red Consulting for being 100% prepared for this PayPal change!