ofShader::load vs ofShader::setupShaderFromFile

When I try to substitute the line

shader.load("shadersGL3/shader");

with

shader.setupShaderFromFile(GL_FRAGMENT_SHADER, "shadersGL3/shader.frag");
shader.setupShaderFromFile(GL_VERTEX_SHADER, "shadersGL3/shader.vert");
shader.linkProgram();

the program tends to not work anymore. Why does that always happen? I want to use this because then I can reuse the same standard pass vertex shader and only change the fragment shaders for post processing effects. The above code sample is from the simpleTexture tutorial on github, which I tested, and it also doesn't work. Any ideas?