Extruder_2 Stepper Motor does not work
The next logical step is to swap the working board with the one that powers the misbehaving motor. If this fixes the motor but makes the other one stop working, it hints that the stepper driver board might be broken. If it does not fix the motor but the right motor works again, while the left is not, then the motors or the board controlling the stepper drivers might be broken.

As changing the driver boards did (as OP said) swap the problem from the left to the right extruder, it is extremely likely that the board that was originally left is defective in some way.

Getting a replacement part might be in order, and until then, running with a single extruder.