what os are you running after effects on ?
What layer did you put that matte choker on?
How is AE interpreting the alpha channel of your render?
Also, what version of AE are you using? EXR handling is better in some versions than others. You might consider a TIFF or PNG sequence instead of EXR.
I'm using CS6 on Windows 10. The sequence I'm importing is .exr - I'd really like to keep the file format if possible.
I Googled how After Effects interprets the alpha channel... and it is interpreting the alpha channel as 'Premultiplied - Matted with colour'. I'm talking about the main render pass rather than the ambient occlusion pass. In some shots I can remove the the white border by changing the alpha to 'Ignore' but this will only work on shots which have a black background. Setting the alpha to 'Straight unmatted' doesn't solve anything.
In other shots there are some animated stars streaking across the frame, behind the main render pass. In this case setting the alpha to 'Ignore' won't work, that will just black out the stars which is no good at all! The white border is definitely on the render pass, just wish there was some kind of Photoshop 'defringe' option.
Ignore the Alpha, compose your layers, re-introduce the Alpha by using it as a Alpha Stencil/ Alpha Matte.
Thanks, that did it! Admittedly I also applied a Matte Choker to the separate alpha mask as there seemed to be a small white border around small parts of 3D geometry.
The problem is likely due to how the file was rendered from the 3d app. It likely rendered it as matted with a color. You want to render it straight instead.