@Teszla: Say,that you are applying KVL in clockwise direction.
Say,Vc is voltage at 1 terminal of current source & other is GND.The voltage drop(VR) across R is 10 V,voltage at positive terminal of battery(Vv) is 10 V.
So,in KVL, you are doing error in polarity of voltages in equation.
You seem to going in anti-clockwise loop in your KVL the polarity of current source,you're understanding it wrong although you're equation is correct.
Even,assume if your answer of -20 V is correct,your answer implies that current of 2 A is flowing from a node at -20 V to a node at 10 V !!!!!!!!!
Current always flows from a point of higher potential to a point of lower potential.