Access denied after plugin update

After updating the plugin, a customer told me that when clicking “View invoice” in their Dashboard, the page showed a 404 error. I created the invoice again just in case something was wrong, and now all I get is an “access denied” message.

The order is completed and paid, and it doesn’t matter how many times I create the PDF invoice, it gives him that error, like he can’t access it.

This is the url it redirects to: https://sinkra.com/wp-admin/admin-ajax.php?bewpi_action=view&post=3339&nonce=25100dd5f5

In the back-end though, I can see it and I can download it from the order details. The URL seems to redirect alright.

What am I doing wrong? How can this be fixed? It was looking good with the previous version. I went back to the previous one and now it shows the same error. Could it be WordPress 4.7.1. version?

Thanks.