This is related to updates to the Dropbox system that applies to new authorizations. This is something that needs to be supported by the third party app itself. Third party developers received advance notice of these updates so they could update their apps as needed, so please check if there are any updates to the third party app available.
Otherwise, please reach out to the support organization for that third party app for help. If the developers of this third party app are having any trouble with the Dropbox API itself, they can reach out to Dropbox developer support directly with the relevant technical details.