Last week I ran my B5m and one of the motor wires broke.

Now, after soldering on a new wire, I noticed that my car was rolling with a lot of resistance. After further inspection I found out that it was the motor. When I turn the rotor it feels as if the motor is braking constantly (like when you connect the motor wires to each other).

I suspect that the motor got too hot and burned the coating of the stator wires, leading to a winding short circuit. Could this be the reason or is something else amiss?

I did not try to run the motor on the track for fear of damaging the ESC, but it does still work.

Check that there is no solder/wire crossing any 2 of the motor wires where they are soldered on. One strand of wire will make it act like drag brake.

That can't be the reason because this motor doesn't have your typical soldering tabs like a Reedy for example.

I opened up the motor and it looks a bit burned inside. Interestingly, only 2 of the 3 poles on the stator look burned and not the 2 connected to the wire that broke.

Remove all 3 wires and see if it is still has a resistance. It could also be the ESC causing it.

nope, it's not the ESC :/ The motor does it even when not connected and I already tried some other motors with the ESC that worked fine

It sounds like the motor is bad based on your testing.

I agree with Roelof, it maybe the ESC. Breaking a wire on the motor could cause it to loose memory of calibration. The ESC lost it home position when wire broke. It become flaky. Start from beginning of Binding and calibration, just reset. As for heat causing the disconnecting at solder joints, poor soldering and overgearing.

If while the motor is in your hand, NONE of the A B C wires are bridged with solder, and you spin the motor and it feels "tight" or "sticky" it's shorted.

This is essentially how brake is made by shorting out the phases. If it feels like that, sorry to say it's likely bad.