When exporting a video from AFX which is not exactly 1920x1080 some really small bars (1/2px) appear.
If the different size is the height the bars are vertical, if it the different one is the width the bar are horizontal. If both dimension are different the bars are vertical.
I had always use not standard size for my composition and I've never had this issue, so I presume is something related to the last updates of media encoder.
If I export the composition in .mov directly from AFX, the bars doesn't appear.
Are you using a "match source" preset? If not, try that.
Although, make sure that your delivery settings allow for none standard frame sizes. It might be better to confirm the footage to 16-by-9 (1920-by-1080).
Yes it was set up as "match source", the matter is that I need to export the compistion in a format differrent from 16:9.