Note the big red warning, which is what your problem is.
It is called automatically if you echo a View object, or if you do something with the object that requires conversion from object to string. If your controllers return a View object, or a Response object with a View in it, it will be echo'd out in your index.php.
The only way to find out what is causing it, is the code above. Most likely your View object constains a body that can not be converted to string, or contains something that causes the exception to be caught, but in turn causes a further error in the error handler.
The code is the quickest way to determine what is causing the problem.