WordPress (behind HAProxy and Jetty), not multisite, in sub folder – and /en/wp-admin/ keeps redirecting to /wp-admin/

Do you happen to have PHPMyAdmin or Adminer or a similar tool installed in your web server? You could easily run a search & replace. I’d recommend running a search and see if all the matching records make sense. Then, run a search & replace.
As @WebElaine mentioned, WP tends to store the URLs in several places, including some plugins, so it is not always a straight answer.