Cause

To address PLC connections you use an addressing scheme in which a certain configuration value is entered under the format element "Counter" (for example, "Position (slot / module)"). However, In the Properties <...> dialog of the associated PLC box, a non-numerical value is entered in the corresponding field of the PLC structure data tab (e.g. in the Position (slot / module) field).

Solution

In the PLC box enter a purely numeric value in the field, which you also entered as your chosen configuration value under the format element "Counter".