Access to music stored on a server via the "Dropbox" application works indirectly via the Dropbox Desktop Client. If this software is installed on a computer, the Dropbox folder (generated by the application itself) can be enabled for network access.
After that, you can add the Dropbox folder as a music resource to the Teufel Streaming System.
This way can also be tried with similar applications (e.g. Google Drive).