Symptoms
When we execute UMUser.Get Users method it also doesn't return either.
However Get UsrDetail method works.
Diagnoses
The issue was caused by incorrect value in LDAP connection string for this domain.
Resolution
We were able to resolve the issue with URM SmartObject not working for Get Groups method. We found incorrect values in LDAP string for the Domain.
We updated the Domain setting in Workspace, restarted K2 service and that resolved the issue.