My app (siteorigin-updates) has been experiencing quite a few problems over the last few days. As far as I can tell the issue is related to App Engine connecting to Cloud Storage. I'm using the PHP runtime.
Calls to functions like readfile and opendir/readdir are sometimes not working properly for gs:// based files and directories - these functions empty values.
The issue comes and goes. At the moment it's working fine. It seemed to help creating a new version of my app (with roughly the same code). Not sure if that's a long term fix though.