QGLFramebufferObject and shaders, can't get anything visible!

I'm trying to render a scene to a QGLFramebufferObject and use it as a texture on my 3D-object. Everything displays fine when I use old fixed graphics pipeline without shaders, but when I try to do this on a shader-based application, I get only a blank screen.

Does anyone know what could be the cause of this?

This is what obviously causes the blank screen:

Code:

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QPainter painter(buffer);
scene.render(&painter);
painter.end();

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Do I need to reset something related to drawing or what?