I get a "Destination unreacheable" IMCP error. It is when the receiver is trying to access UDP port 18678 on my PC.

It isn't actually the Receiver trying to access the PC, it is the Audio Receiver Manager software on the PC trying to access the Receiver. It polls the Receiver on UDP port 18678 every few seconds to see what track it is playing.

You are probably seeing "Destination unreacheable" returned from the Receiver because the boot process has failed for whatever reason and it is therefore not running the player software which is what responds to the 18678 poll.

Either your intial DHCP transaction is failing to set the right data for the Receiver (so it can't find the PC) or something on your PC is stopping the Receiver from loading it's software over the network.