OpenGL Error 1281 in fragment shader (only by using block interface)

I have a very confusing error somewhere in the vertex or fragment shader. The rendering works when I do not actively use the block interface in the fragment shader. But when I am using it error 1281 occurs. Below you see the working vertex and fragment shaders. The error is discusse below the code.

Vertex shader:

#version 430

in layout (location = 0) vec3 position;
in layout (location = 1) vec4 color;
in layout (location = 2) vec3 normal;
in layout (location = 3) vec2 uv;
in layout (location = 4) vec3 tangent;
in layout (location = 5) int materialId;

uniform mat4 pr_matrix;
uniform mat4 vw_matrix = mat4(1.0);
uniform mat4 ml_matrix = mat4(1.0);


out VS_OUT {
    vec4 color;
    vec2 texture_coordinates;
    vec3 normal;
    vec3 tangent;
    vec3 binormal;
    vec3 worldPos;
    int materialIdOut;
} vs_out;

out vec4 colorOut;

void main()
{
    vs_out.color = color;
    vs_out.texture_coordinates = uv;        
    mat3 normalMatrix = transpose ( inverse ( mat3 ( ml_matrix )));
    vs_out.normal = normalize ( normalMatrix * normalize ( normal ));
    gl_Position = ( pr_matrix * vw_matrix * ml_matrix ) * vec4 ( position, 1.0);

    colorOut = vec4(vs_out.normal,1.0);
}

WORKING fragment shader (normal is used as color):

#version 430


uniform vec3 cameraPos;
uniform mat4 ml_matrix;
uniform mat4 vw_matrix;
uniform sampler2D texSlots[32];

in VS_OUT {
    vec4 color;
    vec2 texture_coordinates;
    vec3 normal;
    vec3 tangent;
    vec3 binormal;
    vec3 worldPos;
    int materialIdOut;
} fs_in;


out vec4 gl_FragColor;
in vec4 colorOut;

void main()
{
    gl_FragColor = colorOut;
}

If I am trying to use the block interface in the fragment shader like

gl_FragColor = fs_in.color;

or

gl_FragColor = vec4(fs_in.normal,1.0);

the error 1281 occurs. I really do not see why this is not working.

EDIT: solution:

there where in fact two problems:

1. int was interpreted as float From the application I send the material attribute to the shader by using glVertexAttribPointer(SHADER_MATERIAL_INDEX, 1, GL_INT, ...); which leads to the problem, that attribute was interpreted as float in the shader By using glVertexAttribIPointerEXT(SHADER_MATERIAL_INDEX, 1, GL_INT, ...); we can overcome this problem

2. The program linking was not successful "error c5215 integer varying must be flat" which leads to a change in the vertex and fragment shader.

   out VS_OUT { vec4 color; vec2 texture_coordinates; vec3 normal; vec3 tangent; vec3 binormal; vec3 worldPos; flat int materialIdOut; } vs_out;

and

in VS_OUT {
    vec4 color;
    vec2 texture_coordinates;
    vec3 normal;
    vec3 tangent;
    vec3 binormal;
    vec3 worldPos;
    flat int materialIdOut;
} fs_in;

Now everything works perfect