*NOTE*
According to Microsoft this issue was resolved with Rollup 4 for Exchange 2007 SP1. If you have installed this update, do not perform this work around.
Recently I was working with an Exchange 2007 setup where Backup Exec was giving me the generic error, “unable to attach to a resource” when trying to do a GRT restore to an Exchange 2007 mailbox.
Now there are hundreds of issues out there when you search on this error. Here is what happened to me:
The environment previously had an all in one Exchange 2007 server. To stabilize we pulled the mailbox role off leaving a CAS/HUB server running on 2003 Server and a Mailbox Server running on 2008 Server. Prior to this the customer was able to perform GRT restores without problems.
The issue started because 2008 Server likes to use IPv6 as a preferred protocol over IPv4. Exchange 2007 listens on ports 6001, 6002, and 6004 on the IPv4 stack, but only listens to ports 6001 and 6002 on the IPv6 stack.
The DSProxy listens on port 6004 and is not listening on IPv6. So ::1:6004 goes unanswered. You can work around this by adding the following entries to the host file of the server to force it to see itself:
127.0.0.1 localhost
::1 localhost
192.168.x.xxx hostname
192.168.x.xxx FQDN
Restart Backup Exec services and try restore again.
See Symantec documentation here.
Posted by halo76 
