@ketre try recalculating normals, (In edit mode select all with A then press SHIFT+N) and I can’t see from the image make sure there is no face on the X axis where the two halves of your mirror modifier meet. Also I think you may have some geometry on the wrong side of the X axis which will cause a problem, but it’s hard to tell from this angle.
Try that, if not as @dostovel suggested can you share the .blend.
