When you change the trigger condition of a workflow, in the workflow designer, the following error message occurs:
"An unhandled exception occurred: The Index was out of range. It must be non-negative and less than the listing.
Parameter name: index

To resolve this behavior, please install the Hotfix 11 for the DocuWare version 6.1.
The Hotfix can be found here:
After using the operator "has changed" it is no longer possible to edit the trigger conditions - DocuWare 6.1 Hotfix 11