I am not sure if anyone has a better solution, but I had the same issue once we upgrade wordpress for a client to the newest version. I modified the file
wp-custom-register-login/includes/class-wp-custom-register-login-loader.php
I added the following 2 line blocks. Right below line 42 (protected $filters;)
I added
/**
* The array of shortcodes registered with WordPress.
*
* @since 1.0.0
* @access protected
* @var array $shortcodes The shortcodes registered with WordPress to fire when the plugin loads.
*/
protected $shortcodes;
Then in the constructor which is now line 58 is you added the above code right below $this->filters = array();
I added
$this->shortcodes = array();
Adding the above code got the site I had to work. I am not 100% sure what it does to the plugin.