SSO error with multiple WFEs

We have implemented SSO with ADFS2. We also have 2 WFEs in our farm.

It works fine as long as only 1 WFE is up and other WFE is down (out of NLB). When we start both the WFEs we start getting SSO errors. The log files on ADFS server reveal that SharePoint is making consicutive requests (6 or more) in a second. Due to which ADFS denies the request and causes SSO error.

When we enabled WIF trace on our WFEs we found that when there is single WFE and everything is working fine (ClaimsPrincipal is created, FedAuth Cookie size is average, SessionSecurityToken is created from it) but when there are multiple WFEs we are observerd that cookie size is really small, validating session token is of type SPDummySessionSecurityToken (Microsoft.SharePoint.IdentityModel.SPDummySessionSecurityToken) and NOT a SessionSecurityToken.

We tried finding out details about SPDummySessionSecurityToken but we could not find any information related to this class.

Please note: SSO with ADFS2 is already implemented in 2 other environments for same application and its working without any problem. It's only one enviroment where we are facing this problem.

Has anyone faced this problem? If you have any clues / suggestions why this could be happening please share.

Thanks & Regards, - Vivek Athalye