In case you are facing workflow errors in the apps "Create Supplier Invoices" or "Supplier Invoices List" (see below for an example) and you would like to get further insight in the error message, please follow the instructions in the SAP Note 2658950. Please also have a look at the online documentation at help.sap.com for flexible workflow of supplier invoices.
In order to analyze such issues an administrator has a set of workflow related apps (e.g. "Workflows in Status Error"). In many cases the reason for "Error in workflow processing" is a missing responsible user for profit centers, cost centers, WBS elements or internal orders.