One bad feature I've noticed with these motors is that the screws that keep the brush holders in place(as shown in your last photo) also carry all of the current to the brush holders and brushes. The screws are made of steel(not a good conductor) and small in diameter. I suspect in your case the screws heated up, burning and melting the plastic support plate and aluminum brush holders. It might be good to put in larger, better conducting screws. Or, somehow extend the lead wires on back of the support plate to bolt directly to the brush holders and brush shunt wires. At the same time, it would be a good time to increase the cross sectional size of the lead wires to the brushes. They look a little undersized to me on the ME motors I've looked at. Also, the connections need to be kept clean and tight.
How do the commutator and the rotor windings look? Also, if there are burrs or dings on the inside of the extruded aluminum brush holders, where the brushes slide, the brushes will hang-up and not have good contact with the commutator. A little clean-up with a file will take care of the problem.
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