GCP IAP There was a problem with your request. Error code 11

Question

I configured Google Cloud IAP for AppEngine, and then, when I try to access my app, I login to my google account, and I am redirected to this url:

project-id.appspot.com/_gcp_gatekeeper/authenticate?state=long-state-string&scope=email%20openid%20https://www.googleapis.com/auth/userinfo.email&authuser=2&hd=domain.com&session_state=a23b09e8ae90697d931d96068fdcf163a7a71010..9268&prompt=consent

This page just shows the following error, and nothing else matters:

There was a problem with your request. Error code 11

Tried looking for logs everywhere but in vain. @Matthew Sachs Do you have any idea ?

Answer 1

Experienced this when setting up a new project and the solution was to turn IAP off and then back on again.

Not a great answer but hopefully will help.