Silly situation solved here. 😜
I created a CPT with archive page. Then trying to create a template to render the archive page, right. Another normal day in the life of an WP and oxygen developer, isn’t it?
Well no, the editor refused to render. “Failed to load template error 500.”
Permalinks, shortcodes, css cache, changing cpt slug in case of conflict, nothing worked.
After pulling my hair and banging my head on the walls for 30 min (few dozen of swearing included) I found out that selecting the post type is not enough, you need also to click the checkbox “Post Type”, see the picture below🤯🤯🤯. Lukewarm water discovered here.😅😅
I hope this help the next poor soul in this situation.