Conditional logic "ERROR: One or more fields have been deleted which are required by this condition."

Trying to set a conditional logic entry, when I go to the conditions display page, it gives me the answer shown above. 

How can I tell what it is that has been deleted, in order to fix this? 

I don't recall deleting any fields, since the form it was cloned from is working just fine. 

The error is about the option that triggers the condition, you may edit the condition in order to see the missing option:

If you have questions, let us know.