This is annoying, knowing I have been able to do it a few days before (with the same machine and same display monitor). I checked and indeed, I managed to make an animation using a custom floating viewport 3840x2160px. I placed the floating viewport in a corner with a saved named view, so I was able to work with standard views and with a smaller viewport with the latter named view. So for captures and animations, I just had to select the top-left corner.