Google Cloud / App Engine (Node JS) "Error: EIO: i/o error, read"

We have a Node JS app deployed on Google Cloud/App Engine. The app just started reporting an IO Error:

Error: EIO: i/o error, read
   at .Object.readSync ( fs.js:617 )
   at .tryReadSync ( fs.js:382 )
   at .Object.readFileSync ( fs.js:411 )
   at Object.Module._extensions..js ( internal/modules/cjs/loader.js:1112 )
   ...

Here's what I will try to do:

• Restart the App Engine instances/servers https://cloud.google.com/billing/docs/how-to/restart-services
• Delete node_modules folders and re-install https://github.com/npm/npm/issues/17030

I will update the question with any findings.