Cause

One and the same value was entered at the Safety address property at at least two PLC boxes that are safety-relevant in the project.

Solution

1. Locate the PLC boxes in the schematic using the Go to (graphic) and Go to 2nd coordinate functionalities from the popup menu of the Message management dialog.
2. Open the property dialog of each PLC box one after the other.
3. Bring the first tab to the front.
4. Enter a unique value in the list of properties in the Safety address field.
5. Click [OK].
6. Then start a new check run.