Ok, I managed to convert manually these document following AmberV’s suggestions from an old post (Formatting Issue?? - #5 by kewms)
Still, I hope to learn how to avoid that in the future.
Blockquote
Open TextEdit and set it to plain-text mode, with Format/Make Plain Text.
Copy and paste what you see in Scrivener into this TextEdit window.
Save the document with File/Save…, and be sure to give it a name ending in “.rtf”. The software will complain, saying that this isn’t a normal text file extension. Ignore it by clicking the Use .rtf button.
Now drag that the file you saved in step 3 into Scrivener—it should hopefully be okay at this point.