jond1 Updating the site and adding the Let's Encrypt (LE) certificate is something I already though of, but I didn't think of modifying the DB and those two values, could be a possibility. Anyway, I'm not sure that's the problem as the site should show some kind of wanking in the browser about not having a valid certificate, right? Anyway I'll keep an eye on that. With EasyEngine v4 I just used the update option for adding LE and it worked pretty nicely.
marty I'm starting to realize that that's the problem. Something on ee v4 (something around Redis, I guess) is messing with the database, because I've tried UdraftPlus as @jond1 suggested and after restoring the site, I got the exact same result: the home page loads without CSS styles, but I can't access the admin dashboard or any other single post at all.
I've tried other WP Backup plugins (All-in-one WP Migration, BackWPUp) without luck, so I'll probably will try first the http:// replaces and will analyze also the site with dev options (no CDN or subdomains involved), thanks for the ideas.
The blog is pretty large right now. I started it in 2005 and has over 3,200 posts and a couple thousand images (the htdocs folder is around 2,5 GB , the database dump is currently at 60 MB) in case this gives more insight into the problem.
BTW: After doing the 'wo stack install' the WordOps dashboard is installed at port 22222, but I didn't catch the username and password and don't know how to recover them. This could be useful for me to access phpmyadmin or adminer in case I try to import just the DB from there instead or by command line or plugin. Can you help on this?