Hi Mosr,
may be it is an problem with one of your ABS sensors. I have the same failure with my old Audi80, when i push the abs-out-button the brake works normal again but with no abs. If you have the possibility, check out the failure codes. If you can't find any failures, try to measure each sensor with an oscilloscope. But the car in the air, unplug the sensor and try to measure the sensor while you turn the wheel. if any one of the pictures on the oscilloscope are different, you got the damaged sensor.
I hope i could help you a little bit
Nice wishes
Martin