I think this is case sensitive

src/main/java/com/gmail/nossr50/config/Config.java

@@ -293,7 +293,7 @@ public class Config extends AutoUpdateConfigLoader {
     public double getAnimalsXP() { return config.getDouble("Experience.Combat.Multiplier.Animals", 1.0); }
     public double getWitherSkeletonXP() { return config.getDouble("Experience.Combat.Multiplier.Wither_Skeleton", 4.0); }
 
-    public double getSpawnedMobXpMultiplier() { return config.getDouble("Experience.Gains.MobSpawners.Multiplier", 0.0); }
+    public double getSpawnedMobXpMultiplier() { return config.getDouble("Experience.Gains.Mobspawners.Multiplier", 0.0); }
 
     /* XP Formula Multiplier */
     public int getFormulaMultiplierCurve() { return config.getInt("Experience.Formula.Curve_Modifier", 20); }