In our environment this usually is a Name Resolution issue.
The SQL-Server has to be able to resolve its Hostname to IP and the other way around, if one doesn't work or gives different output the Integ won't work.
Same applies to the CellServer hostname.
Try to use the sql_bar Command on the DB-Server to configure the Integ, sometimes this gives an additional hint on what is wrong.
"sql_bar.exe config -dbuser:<user> '-password:<passwd>' -appsrv:<hostname> -instance:<instance>"