This error is caused when manual changes have been made in your Marktplaats Pro account.
To fix this error you can follow the steps below.
Delete your ads in the Marktplaats API. In order to do this, you create a rule to exclude all of your items and save the rule.
Run the marketplace API.
Clear the marketplace by pressing "Clear API" at the settings section.
Remove the rule created in step 1, and run the API one more time.
When the API is finished the error should be gone.