Symptoms

Unable to access Manage Site Realms form with error:
"Specified cast is not valid"
 

Diagnoses

Looking through the following tables:

Identity].tClaimRealm]
Identity].tClaimRealmIssuer]

there were realms which were not mapped to any issuer.
 

Resolution

After running a SQL script to insert new entries in the iK2].nIdentity].[ClaimRealmIssuer] to include the mapping, this was fixed.