Case Resolved!! :) Our consultant (Rsult) found it. The field was designed to show on a purchase order to which sales order it is connected. It was limited to a list of only sales orders and therefore showed the error code. The workflow that's behind it, is now adjusted and I no longer get an error. Thank you for your…

Hi Pauline, Here some screenshots; Hope this clear things? If you need more info, please let me know. KInd regard, Ron

Thanks @Jeff Pineda-Oracle; I have changed my profile :) @Pauline Joy Cac-Oracle; I have found the field, but it only applys to Purchase and in the Validation & Defaulting tab it says; "Allow and Set Dependent Field Values to Null" I can continue after this error and this is still in Sandbox, but I wouldn't want this error…