MY_Controller not found - once again

If your prefix is Main_, then your base controller needs to be named Main_Controller

thanks, but my controller in application/core is named Main_Controller ...

Are you sure? Earlier you wrote that your controller is named Main_controller, with controller in lowercase. In Windows, this won't cause an error, because Windows is case-insensitive regarding file names. But on Linux, the file name should be Main_Controller.