The map is looking great. That line is pretty weird though. Have you gone through the layers one by one to see if it really is just on the parchment layer? Perhaps try deleting that layer entirely (not just the image on it) and then redo it (unless that is already what you've done)? Alternatively you could try making a new file and then moving all the layers over one by one, although that seems like it would be super laborious. To me that line looks like it is a layer in itself, since it looks like its under the border, perhaps you've got a layer that is cropped to that area and is adding an effect to the parchment. Have you tried replacing the parchment with a different parchment file?