Ok I can provide an update. Today I made a cleanup of the DB, and removed some caching dirs and it seems better now, but I still can´t control the layout that is a bit messed up.
I don´t see most of the changes I make in the settings page.
But I figured out what the issue with the posts was.
I have posts cloned from one subsite to another, and it seems that some metadata is still linked to the original post, I didn´t get which one yet.
Related Posts PRO seems to get somehow confused by these metadata that are probably duplicated, or it filters the duplicate and it opens only the original post, and this created a bit of confusion.
Overall significant improvements, the CSS aspects can be probably related with the structure of Divi that is pretty peculiar.
Looking forward for your analysis and feedback.
Kindest regards
Lorenzo