This was a two step solution for me.
The first was easy. The SQL service account was locked out. DBA unlocked account.
The second step was to uninstall SSL Certificate from the SQL server and reboot. Even if your DBA is not forcing SSL encryption on the SQL server, you may get the error. I hope this helps.
Environment:
Horizon View 6.1
SQL Server: Clustered, SQL 2008 R2