Hello, @timeglas. Thanks for reaching out.
Unfortunately, the Calendar from Google plugin does not allow retrieving the
list of users. It only has the following functions:

This error usually appears when the URL of the page from which you are authorizing to Google is not saved into your project’s Redirect URIs list on the Google Console. How does it look, for example:

You just need to insert here all URLs (with a debugger, with version-test inside) of the page you are authorizing to Google. That should help.
Please let me know if there is anything else I can help you with.
Take care. 