Can you add this to your themes functions.php and then navigate to admin area to see what message you get?
function debug() {
if(function_exists('is_plugin_active')){
echo 'function exits';
if ( is_plugin_active( 'redux-framework/redux-framework.php' ) ) {
echo 'redux is active';
}else echo 'redux is inactive';
}else echo 'function not exits';
}
add_action( 'admin_head', 'debug' );
Is hard to debug like this.
