Daily Demand Plan CSV Import Error - Invalid Form

Hello - we are having issues when attempting to CSV import Daily demand plans for items in our Sandbox. We got through all the date and formatting errors, but now are getting the following error when trying to import a demand plan for any item (assembly or inventory). The error is: "Invalid customform reference key -600."

Currently, I cannot find that custom form with ID -600 anywhere. I can customize the Manual Item Demand plan form, but once I save it, the custom form does not show up under Entry, Transaction, or Other Record forms anywhere.

My question: am I missing where these forms are stored? Why are we getting an invalid customform error when we haven't customized the form in our system? There is not a place to select a form in the CSV import Advanced Options either.