Dropbox deprecated the original version of their API (Application Programming Interface) in June 2016 and shut off this old interface on June 28, 2017. This API is what allows OnSong to communicate with the Dropbox service. So in the end, this means that older versions of apps that use this older interface have stopped working.
Please be sure you are using the latest version of OnSong to make sure that Dropbox continues to operate. You can learn more about the changes in Dropbox in this video: http://onsongapp.com/videos/dropbox
You can learn more about how to update OnSong here: http://onsongapp.com/videos/update