Unable to translate 2D screen coordinates to 3D

Hi there,
I have a 3D openGL scene which I render then I am able to translate mouse position into the exact 3D shape thanks to gluUnproject.

However after I tried to add post processing effect (scene blurring) to the rendering code, gluUnproject gives me unexpected values.

Note that the post processing effect affect the whole scene.
Trying to call gluUnproject previous to the post processing effect and saving the result does not help : it is trhe same issue which bizarre (it is like gluUnproject needs the final SwapBuffers to work)

Unless there is a possible way to make it work, I would like to know if there are other ways to obtain the 3D coordiantes of the mouse position other than gluUnproject.

Thank you in advance.