Well, I've found a workaround. My project was created as "DV PAL". I have created some sample projects with other type of project (VCD, SVCD,...) and always get the line at the right of a simple title, except for the Square PAL project, which works fine, so I've saved my old project as ASCII, edited it with gedit and changed the DV PAL for SQUARE PAL. Then opened it with openshot and... no vertical line.
I don't know if SQUARE PAL is worst than DV PAL but that molest line has gone!