Solved the problem.

It turned out one of the screws on the power-in on the motherboard went loose from vibration. Increased resistance fried the connector. Replaced connector, retightened screws, works as new.

In the mean time got similar issue with one of the extruder heaters, probably one of the wires got desoldered. Cable management on those printers is hell incarnate.

Still, the printer is now at over 6K hours of print, works like a charm. They are mean, angry, reliable workhorses if you know how to take care of them.