Float texture CopyTexImage2D problems

Currently I am rendering to a float pbuffer (resolution: 256256 pixels) with a float texture(512512). I am using a fragment program to sum four texels of the texture to one pixel of the output.
I am getting nice results. (glReadPixels reads the pixel data nicely, result is as it is expected.) However if I try to copy the result to a 256256 texture (with glCopyTexImage2D), I get no results whatsoever. (texel values are 0.0)
I have tried copying the pixels I obtained with glTexImage2D and I had the expected result.
The catch is: the original 512
512 texture was created with the same glCopyTexImage2D function on a 512*512 float pbuffer!

Do you have any idea what could be wrong?

Yours sincerely: Blade