Big change is probably the OS Win 2003 -> Win 2012 (change of SQL Server may have compatibility issues but they are more likely to show after connect because error you are getting is saying it ca't connect so at that point the application won't even know what server version it is.

My guess is it may be something simple like the Win 2012 firewall blocking connections or the protocol not enabled on with new SQL server - make sure you can connect to the new server and db from chosen client with ISQL using the same credentials etc.