The plugin defines WP_DEBUG as 'true' in simplr_reg_page.php, without checking to see if it's been defined already.
Firstly, a plugin should not define WP_DEBUG by default. Secondly, it shouldn't define it even if it has already been defined!
I assume it's just some test code that was left in, but could it please be removed?
Cheers, keep up the great work!