Now it works

FightSystem_API/src/de/steamwar/fightsystem/utils/Region.java

@@ -144,7 +144,7 @@ public class Region {
     }
 
     public boolean inRegion(Block block){
-        return in2dRegion(block) && block.getY() >= minY - 1 && block.getY() < maxY;
+        return in2dRegion(block) && minY <= block.getY() && block.getY() < maxY;
     }
 
     public interface TriConsumer<T, V, U>{

FightSystem_Main/src/de/steamwar/fightsystem/listener/PistonListener.java

@@ -39,9 +39,8 @@ public class PistonListener implements Listener {
 
     @EventHandler
     public void handlePistonExtend(BlockPistonExtendEvent e){
-        BlockFace b = e.getDirection().getOppositeFace();
         for(Block block : e.getBlocks()){
-            if(!Config.BlueExtendRegion.inRegion(block.getRelative(b)) && !Config.RedExtendRegion.inRegion(block.getRelative(b))){
+            if(!Config.BlueExtendRegion.inRegion(block) && !Config.RedExtendRegion.inRegion(block)){
                 e.setCancelled(true);
                 return;
             }
@@ -50,9 +49,8 @@ public class PistonListener implements Listener {
 
     @EventHandler
     public void handlePistonRetract(BlockPistonRetractEvent e){
-        BlockFace b = e.getDirection().getOppositeFace();
         for(Block block : e.getBlocks()){
-            if(!Config.BlueExtendRegion.inRegion(block.getRelative(b)) && !Config.RedExtendRegion.inRegion(block.getRelative(b))) {
+            if(!Config.BlueExtendRegion.inRegion(block) && !Config.RedExtendRegion.inRegion(block)) {
                 e.setCancelled(true);
                 return;
             }