grayscale shader on iOS turns black

hey everyone,

i've got a bit of a shader problem. Since i have no experience with shader-coding (or programing in general), i hope someone will be able to help me here.

It's a simple grayscale shader, I got the code somewhere off the internet. Nothing special.It's applied to a material and everything's regular. The material is applied to a sprite.

<- with default sprite shader <- on PC <- on iOS

Now on mac, or win, everythings normal and how it should be. Grayscaled. On my iPad otherwise, it's just black. pure evil. I have no idea why. Here's the shader-code.

 Shader "GrayscaleLolTransparent" {
     Properties {
         _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
         _EffectAmount ("Effect Amount", Range (0, 1)) = 1.0
     }
  
     SubShader {
         Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
         LOD 200
  
         CGPROGRAM
         #pragma surface surf Lambert alpha
  
             sampler2D _MainTex;
             uniform float _EffectAmount;            
  
             struct Input {
                 float2 uv_MainTex;
             };
  
             void surf (Input IN, inout SurfaceOutput o) {
                 half4 c = tex2D(_MainTex, IN.uv_MainTex);
                  o.Albedo = lerp(c.rgb, dot(c.rgb, float3(0.3, 0.6, 0.2)), _EffectAmount);
                 o.Alpha = c.a;
             }
  
         ENDCG
     }
  
     Fallback "Transparent/VertexLit"
 }

I haven't found anything in the search function. Just hints, that it may be because of some GLES shader stuff... I don't know.

Any help?

Thanks in advance!

cheers