[SOLVED] Update.php Script - Page not Found

I do not have your answer
But I know how to use Google and limit the search to drupal.org
since the d.o website's search is essentially worthless to me
except maybe when I am in a specific project's
(module's or theme's) issues que.

I suggest a Google Search limited to drupal.org

Google Search: site:drupal.org
https://www.google.com/webhp?q=site%3Adrupal.org+ 

• Do Not insert a blank-space after the colon (":") before the first letter of the domain name ("drupal.org").
• Type a blank-space following the plus-sign (+),
  and add your keyword(s) &/or "keyword phrase"(s).
• Wrap a multi-word keyword phrase in double-quotes ("... ... ... ")
  to search for exact phrase matching in the search.

Sample Google search of d.o

Example using keyword(s): can't run update.php
https://www.google.com/search?q=site%3Adrupal.org+can't+run+update.php
.
.

Make sure your PHP version is 5.2! Mind your PHP version. I know that Drupal 7 should work with PHP 5.3 (I read somewhere that it's even recomended), but in my personal experience, it blew up the website in a specific host.

With PHP 5.3, Drupal 7.19 returns "/update.php not found" on multisites (and I believe it does the same on single-instalation sites).
It may also give an occasional "500 - Internal server error".

I don't know the cause, but this is what happened to one of my sites, hosted at netfirms.com.
In other hosts, and in localhost, it worked fine with other versions of php 5.3.

Hope it helps.

«UPDATE»
I finally solved my issue, and turns out it had to do with the PHP upgrade, CGI and php.ini configuration.

it seems that, for php versions after 5.3.10, if php is running as CGI, and it uses force-cgiredirects, a couple of changes should be done on your php.ini. The following is an extract available at http://php.net/security.cgi-bin:

This is what I found out while trying to get php to work as CGI with Apache VirtualHosts.

By enabling 'force-cgiredirects', you *must*:
1) set 'cgi.fix_pathinfo=1' in php.ini
2) leave doc_root commented out (php.ini also)

If you miss item 1, the apache logs will show 'unexpected T_STRING' in the php binary.
If you miss item 2, you'll only see 'No input file specified.', instead of the expected output.

You can then turn on the php support for a particular vhost by defining:

Action php-script /cgi-bin/php

inside the corresponding directive.

Even so it wasn't enough, I had to enable authorize.php and update.php permissions to owner execute.
After that, it worked like a charm.

Hope it helps someone with a similar problem.

One possible solution, which worked for me was

The permissions on update.php should not allow group write. The permissions were 0664. They should be 0644.

From http://drupal.org/node/1010730

For me the solution was also to update / remove the boost code in .htaccess.