Business rule executed unsuccessfully due to declare [] field

When you created a business rule and see an error like this, please check your settings in the action panel:

"Custom rule executed unsuccessfully [...]: Neither [ActivityDTO_V21] class, nor its super class declare [] field"

You don't need to define the object type in the name of the fields. If you do so, the error above will be shown, because the field is empty:

NOT Correct:

Correct: