Unity 5 - Shader error in VertexBlend : Too many output registers declared (12). How to fix it?

Hi! I upgraded a project to Unity 5, I had the usual " variable not fully initialized" error in a lot of shader, I fixed it by adding UNITY_INITIALIZE_OUTPUT(Input, o); No problem. But now I have a shader that blends 2 * (diffuse+normal) texture pairs based on a blend texture I got a strange error. The shader only throws the error when in playmode. The shader error:

 Shader error in 'VertexBlend': Too many output registers declared (12).  When no output register has been declared with the semantic 'psize0', 11 o# registers are available.  When an o# register has been declared with the semantic 'psize0' (same as 'psize'), 12 registers are available. at line 192 (on d3d9)
 
 Compiling Vertex program with DIRECTIONAL SHADOWS_SCREEN LIGHTMAP_OFF DIRLIGHTMAP_OFF DYNAMICLIGHTMAP_OFF FOG_EXP2 VERTEXLIGHT_ON

It seems to work, but I am affraid it will break on some mobile devices. Can anyone tall me what is wrong? What should I change to make it work?

My vert output structure:

 struct Input {
 float2 uv_MainTex : TEXCOORD0;
 float2 uv_Diffuse2 : TEXCOORD1;
 float2 uv_BlendMask : TEXCOORD2;
 float4 color : COLOR;
 float2 uv_Normal1 : TEXCOORD3;
 float2 uv_Normal2 : TEXCOORD4;
             };

Thanks!