Background video in "Active Shade Viewport mode" doesn't match with Viewport, Active Shade (window mode) and final Rendering ?

[patrick.proier]

Hi!
I do a 3D tracked VFX shot in 3ds Max with Arnold and Tyflow and I have absolute no idea why the background video (Environment) looks in Active Shade (viewport mode) different (wrong aspect ratio) from the Active Shade window mode.
My 3D Objects match perfectly in the Viewport, in the Active Shade (window mode) and in the final Rendering only in the Active Shade Viewport mode they are off
I have "show save frame" activated but the background proportions change when I change the aspect ratio of my Viewport in the Active Shade viewport mode.

Any ideas? I never had that problem in Maya.