Solution: You should check the EVR filter settings first:

Make sure the image has not been moved and has been extended to the whole field preview, it should look like this:

If the settings are correct and the picture is still displayed only on part of the screen and you have a GeForce graphics card (nVidia), go to the NVIDIA Control Panel -> 3D Settings -> Adjust picture and select "Use the 3D application setting" (be sure to confirm with "Apply settings" button).