Long 24V DC cable not working
One issue I have had in a similar situation (An underwater electronics pod on a few tens of meters of cable with power and RS485 control wiring), the fault was due to the voltage drop in the ground connection....

Essentially the IR losses in the supply negative connection (actually the cable screen) were sufficient that the control signals from the head were outside the RS485 common mode range of +12,-7V. Going to an isolated RS485 port at one end fixed it. 

One other gotcha with DC/DC bricks like that buck/boost you have is that they sometimes misbehave if the supply is insufficiently stiff on startup, try adding a fairly large electrolytic across the DC/DCs input to stiffen the supply.