Typically this error points to the communication failure to the client and that something is blocking the Multicast traffic. Are you certain that the firewall ports are open for Multicast on the clients?
| TCP | 33354 |   | Software Distribution (Peer Download, Multicast) |
| UDP | 33354 |  | Software Distribution (Multicast) |
| UDP | 33355 | | Software Distribution (Multicast) |
I'm assuming the ports on the core are ok since some multicast clients are handling this successfuly. You also need IGMP enabled on the router\switch.