Dropbox Saver suddenly stops working

I have a membership site in which I use the Dropbox Saver alongside the Google Drive saver button to allow members to download files. 

It's been working great over the last few years, but today, suddenly, without me making any changes, the dropbox saver button suddenly stopped working across the site. 

The Google Drive button for the same file is working fine, and I can download the linked file by right clicking on the saver button and selecting "save target as."

I've tried changing the app, setting the access token expiration to never expire, I've tried changing permissions and nothing seems to resolve it. 

Further, if I create a new button for a new file, the button works fine, but if I then change that same button to a file previously linked by another saver button it doesn't work. 

Anyone else come across this, or can point me to how I can work out what the problem is?