"breadcrumb_navxt_admin.php" does not exist

You just need to reactivate the plugin, see the sticky thread in the Breadcrumb NavXT support forums: https://www.ads-software.com/support/topic/readme-before-updating-to-500?replies=1

The file containing the plugin header changed in 5.0.0. Unfortunately, WordPress does not provide a method for easily migrating the file with the plugin header (or rather the plugin updater in WordPress has a bug).

Thanks – this helped me too.