For those experiencing the WordPress 5.2 breaks admin issue, restoring /wp-admin, /wp-includes, and wp-settings.php has helped get the admin system back online.
If you don’t have a backup, you can pull these from the original 5.1.1 WordPress installer package. For safety’s sake, I’d rename each instead of replacing — e.g. rename /wp-admin to /wp-admin_5.2, etc.
Out of a few dozen websites I manually upgraded, only one experienced this issue, but based on others’ experiences, it looks like it’s something plenty of people are seeing. After updating to 5.2, the website remained online, but the wp-admin panel was inaccessible and you’d get an err_too_many_redirects error.
Just thought I’d put out a way that got the WordPress admin back online.