Symptoms
No realm entries for 'https://k2.domain.com/ViewFlow/' was found.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Configuration.ConfigurationErrorsException: No realm entries for 'https://k2.domain.com/ViewFlow/' was found.
Diagnoses
It seems like the configuration in the [Identity].[ClaimRealm] and [Identity].[ClaimRealmIssuer] tables is wrong.
We couldn't access these entries through the normal way by browsing to K2 Designer -> All Items -> System -> Management -> Security -> Forms -> Manage Site Realms
Resolution
We found that the table [Identity].[ClaimRealm] was correct with all the realms, however the table [Identity].[ClaimRealmIssuer] had an entry missing. So we just added a new entry to the [Identity].[ClaimRealmIssuer] table, to link the Realm with ID = 5 to the Issuer with ID = 1 (which is WindowsSTS).
As soon as we did that, K2 knew to use the WindowsSTS issuer to open that Realm Entry and could open the ViewFlow successfully.