Symptoms
After upgrading the production SQL instance from SQL Server 2008 to SQL Server 2012, the user was receiving the following error when trying to login to Designer:
Cannot open database "[DATABASE NAME]" requested by the login. The login failed.
Login failed for user '[SERVICE ACCOUNT]'.
Diagnoses
This was most likely a situation in where after the upgrade something related to the service account credentials had changed.
Resolution
Running through the K2 blackpearl setup manager would have updated the invalid credentials within the context of the SQL server.