White Screen When Includding A Folder With PHP Files

By the WP coding guide, please, use WP Plugin API.
In your case place the following code into start of the main .php file of the plugin.

// Working directory definition.
define( 'MY_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );

// Attaching executable
require_once MY_PLUGIN_DIR . 'folder/class-my-class.php';