Symptoms
New user that never use the application was able to load the form successfully. However, if users used the application before, the application will not work.
Diagnoses
The issue was caused by SAP cached credential info in K2 DB can no longer be decrypted by K2 since the Service Master Key was recreated when SQL server was moved.
Resolution
Ran the following SQL command to set the value of passwordcached = 0, so the application will re-cache user's SAP credentials: update securitycredentialcache set passwordcached = 0 where passwordcached = 1 and username = '[SAP Account]'