As Ian mentioned, if you are able to validate the credential using RDP then you know you have a good credential. RDP uses port 3389 which is a different port than ServiceNow Discovery uses.

Once you have a good credential, then I suggest you validate the ServiceNow Discovery WMI ports between the MID Server and the Remote Windows host are open (in the Windows Firewall and/or any firewall between the MID Server and the Windows host). ServiceNow Discovery uses WMI for discovery, therefore port 135 from the MID Server to the Remote Windows host must be open for initial communication AND high ports 49152 - 65535 must be open for the remainder of the communication. Even though this is a large range of open ports, only a portion of this range are dynamically allocated.