No system available - When connecting to backend system

In the VC we have created our models, we then select data, then click on the portal connect button, the logon page is then displayed from which we log-on, the logon page disappears and the VC 'Data' page is displayed, the 'Connect to System' field display's a message 'NO SYSTEMS AVAILABLE'.

We have checked and tested the Portal settings which are all connect the R3.

The VC has previously been working and we have implemented a live application last year, however, we have recently re-built our server and ever since the VC has been displaying the above error message.

We are on EP6 SP2, and the VC version 6

Can anyone please help...