The IDP session cannot be retrieved from the IDP cache on the second OpenAM instance if a user has already completed SAML login on another instance in the same site (in the same browser session).
Steps to reproduce:
1) Configure a site with two instances behind a load balancer with sticky balancing
2) Configure one more standalone server as the SP
3) Configure federation between the site and the server
4) Initiate SAML login on the first server in the IDP site (idpssoinit), enter credentials > "Single sign-on succeeded"
5) Initiate SAML login on the second server in the IDP site (idpssoinit) > "HTTP Status 400 - Error processing AuthnRequest. IDP Session is NULL"