Error 400 generally means that Notability's connection to Google Drive or Dropbox needs to be reset. You can do that by logging out of and back into the service in question through these locations:
- On iPad, go to the Manage Accounts section of Notability's settings.
- On Mac, click Notability > Preferences… from Notability's menu bar.
If this error persists after you've reconnected Notability to the relevant account, feel free to let us know! We'd be happy to investigate. You can get in touch with us by clicking "Submit a request" in the top right corner of the screen.