I modified a 28BYJ-48 to bipolar.
I am using A4988 driver to drive it.
When you first tested your configuration -> did you limit the current of the A4988 being driven through the coils by the voltage reference method?
Unfortunately I can't find a reference to what minimum current you can limit the A4988.
I suspect you might have fried at least one coil of your stepper motor with too much current flowing through it.
Can you measure the resistance of the two coils with a multimeter?
If my guess is right, you will only measure one coil's resistance in the magnitude of some Ohms and the other one either close to zero or "open".
Playing around with the potentiometer on the A4988 is never a good idea - this should only be used with no stepper connected and measuring/adjusting the reference voltage according to the current limiting formula (see Pololu's reference site on the A4988).
Have you checked the resistance of each coil and that the coils are still isolated from each other?
Yes i checked that.
I connected a LED to the coil ends and when I turn the shaft, the LED lights up.
Than, after fiddling with the code a bit I a while ago i was able to make the stepper run in the anticlockwise direction. (chang ein the code is inside the loop()