Mysterious black line appears after render

Hi, I have been using PP for years and have never run into this before: a thin black line appears on the right-hand border of my video and I simply cannot make it go away. If I scale up the video, or move it to the right, it looks fine, but upon render, the line pops back in. I have double-checked all aspect ratio etc. It all squares up. This bit of video is a bit odd, 854x480 with a 1.0 aspect ratio, but even when I make a sequence using custom settings and put exactly the same values, the problem still occurs. Any ideas? Thanks