Symptoms
Users are constantly experiencing the following error on the smart forms. The issue only occurs for 5-7 users. The error states Impersonate Session Security Context. Cannot be called in this batch because a simultaneous batch has called it.
Diagnoses
The issue was caused by missing "User login name" property for these users in AD. They only had filled in the "User logon name (pre-Windows 2000)" field. As a result, the value of "CustomUserID" field for the table "[HostServer].[SecurityCredentialCache]" is set to Null for these users.
Resolution
Updating the "User login name" property for these users and clearing IE Cache resolved the issue.