WebEx - 'Auto Account Creation failed (31)'

[Ryan Suarez]

Trying to federate our shib IdP v2.3.8 with Cisco WebEx SP.  Login with 
an already existing user in WebEx succeeds.  Login with a non-existent 
user in WebEx fails with "Auto Account Creation failed (31)".

Cisco docs provide a sample saml response for auto-account creation:
http://pastebin.com/Fd1ZYx1j

Here's our response (from Firefox/SAML Tracer tool):
http://pastebin.com/7nYRyjWz

I don't know what the problem is.  Is there something missing from our 
response?