400 Bad Request using Google Calendar synchroniser

[Mavyre]

Describe the bug
When trying to synchronise, the API throws 400: Invalid email or User ID.

Client error: `POST https://oauth2.googleapis.com/token` resulted in a `400 Bad Request` response: { 'error': 'invalid_grant', 'error_description': 'Invalid email or User ID' }

To Reproduce
Steps to reproduce the behavior:

1. Create a service account
2. Download JSON key
3. Put the Email account and JSON key into the "Synchronise to Google Agenda" tab
4. Run one of the tests

Expected behavior
Should work as service account is up & running

Additional context
Tried with Google Workspace account. Tried any calendar.