Yes, this is the longest downtime I've had on App Engine for years. I've been up all night and the app engine team were up working on it as well and sending updates.
It seems that the default authentication method broke for some apps. They have different APIs to reach your data, and I tried another approach and it worked, but it's different enough, and we use Cloud Storage in too many places, that it would take a day or more to change our code to use it. I started on it anyway, in case the outage lasts longer.
