What might cause "No direct script access allowed" during EE update

Question

I'm in the process of updating a site from EE 2.9.0 to 2.9.3. As always, I've follow through the update process specified in the docs step by step. Upon running the update wizard the process seemed to get to 2.9.2 (ie. it was showing 2.9.2 in the wizard's progress window, but I don't know whether that means it was in the process of installing 2.9.2 or whether it meant that 2.9.2 was fully installed and it was now installing 2.9.3). But rather than completing the process it suddenly displayed a white screen saying "No direct script access allowed". The URL in the browser is http://mydomain/admin.php?C=wizard&M=do_update&language=english

Re-running the wizard gives the same result, except that this time the white screen appears immediately (without showing the wizard's progress window and without mentioning 2.9.1 or 2.9.2).

I've never run into this issue before during updates; does anyone know what might be causing it?

Answer 1

Looks like previous issues like this could relate to your plugins : https://ellislab.com/forums/viewthread/110352/#691061

Perhaps try removing your plugins before upgrade then re-add them?

Further we see this error message amongst many CI and EE files at the top of the class files, it's so common that debugging can cause all sorts of issues:

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

Can I suggest you look to your broken installation and see what the state of the BASEPATH definition is?