In your non-working generate function, you don't set the GL_DEPTH_ATTACHMENT for either of the FBOs. Try setting those and see if that works.
RenderBuffers (including Depth-Buffers are not actually necessary for what I try to do - like i said in the last lines of my first post - i already tried if it was them by commenting them out in the working code which changed nothing on the working status)
Thanks, that was the issue, code-blindness - didn't saw it.
Also, this:glBindFramebuffer(1, m_frontBuffer);
should be this:glBindFramebuffer(GL_FRAMEBUFFER, m_frontBuffer);