Thank you for you quick answer. I checked my Google account and it has still Less Secure Apps enabled. I then created an application password and enabled 2FA (because I couldn’t create the application password without).
After that the test connection still works, but nothing happens when my cronjob trickers the Webhook URL.
What can I do to get it work? Do I have to wait for version 2.0 of your plugin in order to use the oAuth method?