This article provides more information about the syslog message "kernel: %KERN-3: rdp_usr_send(): can't send on disconnected socket" that is seen on MX Series routers.  

The following syslog message is seen to flood the router with no known service impact.

Jul 15 08:05:13.370 2021  router-re0 kernel: %KERN-3: rdp_usr_send(): can't send on disconnected socket

These messages are generated by the kernel. The RDP is an internal Junos OS communication protocol that is used between chassis components such as the Routing Engine and the Packet Forwarding Engine. This communication requires a socket and the above syslog message indicates that communication messages cannot be sent due to a disconnected socket.

The message by itself is harmless and can be considered as informational.

Even though this is an error message, if it is reported in isolation with no other issues or error logs reported, it can be safely ignored. 

However, if there are other issues or error logs reported, it is recommended that customers investigate the other logs, or report the error log/issue to Support for in-depth troubleshooting.