The Section Divider is an element that located inside the section element. Line between Dividers could appear in case there is a space between the sections
At first, make sure that there is no custom space added in Design settings of the the sections in Spacing and Transforms option groups:
Custom CSS code could also cause this issue. You will need to remove all the . custom code from the section settings, page settings and Theme Options to check if the issue is related to any custom code or not.