Unwanted verticle line on fax copies
Most likely a fax has been sent or a page copied with 'white out' (Liquid Paper), still wet on the page. As the page feeds over the scanner, (a narrow strip of glass that runs across the width of the page along the feed path) the wet liquid paper comes off on the glass as a thin line and dries. Thereafter, when copying, scanning or sending a fax, the scanner light does not go through the liquid paper streak and the result when printed is a thin vertical line that runs the full length of the page. If this is the cause you will also find that faxes you send to other people will also have this line (although faxes you receive are not affected by this problem with your machine).
To fix the problem: follow the path that the paper travels when sending a fax or making a copy, until you find the scanner (narrow glass strip). Take a clean, soft cloth and a small amount of methylated spirit (or white spirit) and remove the now dried Liquid Paper from the glass. Clean the whole strip ensuring that the glass is left clean and dry.