I don't know how much this will help as it is from the M600 manual, but that says a 50.4X.YZ error means the machine has detected an over temp issue in the fuser (for what it is worth, the manual helpfully further explains only that "X"= the fuser mode, "Y"=previous printer sleep state, and "Z"=next printer sleep state, but not which each of those means exactly).
If you've ever worked on old Canon fax machines, that experience can be helpful with HP equipment. I mean, I apologize if you've already done this, but you should turn the machine off and unplug it for a while (I don't know, get a cup of coffee or something, that usually works for me, although it could need to be unplugged for around ten minutes so take your time if possible). And maybe check the outlet the machine is plugged into to make sure it's up to spec.
Anyway, back to the manual, such as it is. It says if the fuser is OK then to check the connector for the fuser for damage or not being plugged in correctly, and then replace the low voltage power supply.
Also for what it is worth, here's something I found on HP's support website that might be more helpful than me:
HP Color LaserJet Enterprise M855 Printers - 50.4F.00 Errors | HP® Customer Support
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