I have this strange Deja Vu....
....
As it is brushed there is the possibility that the commutator is worn/damaged or just filthy.
Can you see a lot of sparks coming from that area?
Grab some earbuds and Isoprop and poke them in the hole where the brush goes while you try to turn the motor if that is anyway possible.
Otherwise you need to take it apart, remove the back plate off the motor and inspect and clean the commutator.
There is also the possibility that there is a short in the coils or some bridging between the commutator plates. Sometimes carbon build up creates arcing the can even melt the copper but sometimes this can be reworked skilfully with a needle file or if it is just the carbon build up with a needle and lots of Isoprop.
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