papillouma,
If the lines are gone in PDF and JPEG, it seems you are looking at a screen artifact, and that it will be right in print and in raster formats.
How about SVG, if relevant?
What happens if you tick/untick Anti-Aliased Artwork in Edit>Preferences>General?
Are the lines visible at all zoom levels, af if so, is the width the same or growing with zoom level (same width is also a sign of a screen artifact)?