Why does the PayPal redirect gets stuck on my Wix site?

It could be that you are using the Wix's free site_username.wixsites.com subdomain. Due to a redirect security limit on wixsites.com, the paypal redirect won't work for forms embedded there. If you use a your own domain, you PayPal will work perfectly. The alternative is to link a donate button to your donorbox.org/campaign page.