Homeplug and MoCA are "layer 2" devices. An ethernet switch is (excluding $$$$ ones) are layer 2.
Layer 2 isn't aware of TCP/IP let along DHCP which is a use case of IP.
So the error might occur with straight cat5 to the router LAN port.
Or it could be intermittent (try many times before concluding).
If it seems to be HomePlug dependent, than assuming the Homeplug devices aren't faulty, and assuming the signal strength is OK, and no interference, then I'd blame the router as not doing DHCP correctly for all timing cases, or the DLAN device not doing DHCP correctly, or both This is classic.
The DLAN IP address is just for administering it.. that IP address isn't relevant to packets going to/from the two mediums.