I don’t know why it is not working any more, there has not been any changes to the Dropbox backend for at least a year.
The error message indicates that Dropbox sends a response, but it is not in valid JSON format as expected, which gives the error.
Since the traffic is sent over SSL you cannot simply sniff it to see what actual response you get. Instead, you need to make a configuration file:
https://docs.microsoft.com/en-us/dotnet/framework/network-programming/how-to-configure-network-tracing
Simply save the xml document from that link, and name the file the same as the executable you are using, and add .config (i.e. commandline would use Duplicati.CommandLine.exe.config and TrayIcon would be Duplicati.GUI.TrayIcon.exe.config). Then restart (if using web-ui) and try the request. You should get a file called network.log which contains all the data sent and received.
Important: Don’t post the file anywhere as it contains your OAuth login token!
In that trace file you should be able to see what the Dropbox server respons, and that should give a clue as to what the problem is.