My guess is that there's something wrong with the directory you've set for Sendy and that we aren't able to communicate with their API. I'm not aware of any issues with the Sendy integration itself, so all I can figure is that a setting is wrong somewhere due to some change.
Double-check the Sendy URL in our plugin and all other settings. If that doesn't help, let us know a URL where we can see the module and we'll gladly try to investigate further.
Thanks,
Alex