What options are selected under the usage restrictions for the credential you have configured?
If you do not have "May be used with NTLM Authentication Schemes (Windows NT Lan Manager)" checked then give that a try. Uncheck "May be used with Windows Impersonation for use with RPC" when you make this change as that restriction takes precedence over the NTLM option.
Also make sure there is no firewall in the middle blocking WMI connections.
I tried that and it did not work either, I still received "Access rights are insufficient". However, what I did figure out is this:
1. The monitor works if I use an ipMonitor credential that is linked to the 'builtin' Local Administrator account on the other domain Windows 2008 server, but I am not allowed to do that per our security policy.
2. I was also able to get this to work by DISABLING the other domain Windows 2008 server's Local Policies >> Security Options >> User Account Control: Run all administrators in Admin Approval Mode and then using the local administrator user account I created for ipMonitor to use....But even in this case, I may not be able to configure it this way if our company's securlty policy does not allow me to disable UAC.