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I've had something similiar like this when exporting a .FLV. Just needed a re-encode.
Also I've had my exports consistently fail due to the clock wipe transition, which I had to replace with a different transition.
Well, I know this is pretty late, but I've found this same issue on my timeline today.
I have a keyed clip on the front, on the background there are some images scrolling with a circular mask (with feather) applied to them. If these images are larger than the comp size and I applied a cross dissolve to them, a purple line appears on the right (the size of the line depends on the number of pixels that exceed the comp) of the screen.
I managed this issue removing the cross dissolve and fading the clip from Opacity option under clip Effect controls.
Don't know if this could be helpful for other users.
(sorry for my bad bad english!)