HTTP error 500 while accessing Microsoft Exchange 2013 through NetScaler.
To resolve this issue complete the following changes on LDAP authentication server definition:
Retain account attribute on samAccountName.
Add SSO attribute, set to UserPrincipalName.
Change existing LDAP policy to the new server profile.
Log on to load balanced virtual server with subsequent redirect to AAA virtual server results in credentials automatically passing to Exchange successfully. User is no longer prompted with two different login prompts.