Thank you João for your prompt reply!
Indeed, I only had enabled Google Drive API. I now also enabled Google+ API and the authorization worked fine. I now get a screen on the GoogleSheetsDemo/Authorize with my Google login, the scope https://www.googleapis.com/auth/spreadsheets and last updated date
But the read & write actions are not still working. I get a "403 Forbidden" (module REST (Consume), stack 403 Forbidden at ssGoogleSheetsConnector.CcGoogleSheetsAPI.ActionGetValues).
I insert the Spreadsheet ID as described, but the "Test" button fails with the error above. I've also added the Javascript origin authorization, but it didn't have any effect.
I'm pretty sure I set the correct inputs (Spreadsheet ID and sheet name) on the Demo, I'm the developer of the old Google Spreadsheet API
Any ideas ?
Regards, Olivier