If you move the contents of the folder to which a DropDown List currently points, the List will (of course) become dysfunctional. You will need to repoint the list so that it can 'grab' the files from the new location.
A couple of options exist.
Repoint via the Libraries & Books screen:
If the list reflects the contents of an existing book, you can call up the Document Assembly 'Libraries & Books' screen (the one that appears when you click the Document Assembly button. Click once on the book name. Then click the "Create DropDown List" option. Pathagoras will instantly create the List and ask which position you want the List to occupy. Select the appropriate 'tray' and 'position' of the current list. That is all.
Repoint via the actual DropDown List:
As a practical matter, the repointing function is actually the 'replacement' of the new pointer for the old. Drop down the DropDown List. Scroll down to and click «Other Settings & Actions» at the very bottom of the List.
This is the result"
Click the 'Replace List' button. Follow the steps to navigate to inside the folder at the new location. Select one file from the folder's content (it does not matter which one). Click 'OK' to lock in the folder name and you are done.