This is the setup of accounts and what I tried so far:
Google Account X: Owner of Project XX, Owner of AdSense
BigQuery
> s2s: does not work
> with a Client ID for web applications: no sufficient permission
AdSense Management API
> s2s: does not work
> with a Client ID for web applications: works
Google Account Y: Owner of Project YY, can access AdSense of X
BigQuery
> browser: worked
> s2s: does not work
AdSense Management API
> s2s: does not work
What makes me wonder is: why can I access BigQuery with s2s but then cannot access the adsense database?
And which other authentication can I try? I probably won't be able to access the database too I guess?
Warning: Very few Google APIs currently support Service Accounts. Service accounts are currently supported by the following Google developer services:
Google Cloud Storage
Google Prediction API
Google URL Shortener
Google OAuth 2.0 Authorization Server
Google BigQuery