Tried it several times with seemingly benign configurations, e.g. not meddling with IndexedDB, DOM storage, etc.
Every single time it would trigger 400 when doing XHR to various Google endpoints, people-pa.clients6.google.com being one of them.
This is using new profile, without any addons. An example of prefs.js that does this.
Enterprise Policy has no effect here, the gist above is sufficient to reproduce the issue.