Hello Algaraba, I am the author of Register Plus Redux. I can tell you that that problem with Back-End Instructions is that on line 44 of instructions.php, Back-End Instructions loads pluggable.php. Now, I am not entirely sure why, the preceding line checks for a function 'wp_set_current_user' but I don't see that function actually utilized anywhere in Back-End Instructions. If you are interested in determining whether Back-End Instructions really needs to load pluggable.php you could comment out lines 43 through 45 with preceding // characters, if the plugin still works as expected, then bang, problem solved, otherwise you and/or this plugin's developer will need to review their code to move code that requires pluggable functions further down the page life cycle.
The problem is that plugins are often loaded in alphabetical order. Back-End Instructions is loading pluggable.php which should be loaded after every other plugin. By loading pluggable.php prior to other plugins completing their initialization they cannot create their own pluggable functions, thus breaking the purpose of pluggable.php. This is all rather complicated if you aren't a WordPress plugin developer. Regardless I ran into a similar problem with Maintenance Mode several years ago, the core concepts should still ring true.