Since the target "DART" device is a non-RA device, it's unlikely that RA Tech Support will be able to tell you much about the error code. It's my suspicion that the message is completing but the device is sending an error response to the ControlLogix.
What is this "DART" ? Google has turned up a few suspects.
Try sending a "Device WHO" message to the DART to make sure it's connected and configured correctly to do EtherNet/IP. That is the same function that RSLinx uses to browse device identities, so if the device shows up in RSLinx it will respond to Device WHO.
Having the Service and the Class the same looks suspicious to me, but I'd have to read the EDS file or other product documentation, as Class 0x60 is Vendor-Defined but I don't know of any Service 0x60. Time to go read the CIP Spec a little.