Access Denied 5.1 fresh install

Yes! That did the trick!
I had been scratching my head and cross-eying over gazillion google results to solve this (I'm a total noob), until I found and tried this suggestion. Everything is fine now.

Thank you, thank you, thank you.

Same here. I really could kiss you. I was on the point of throwing my laptop out of the window and starting a new career as a bottlewasher.

I would have sworn I already tried commenting this out as well: I had seriously got to the stage where I was going through all the fixes I'd tried and trying them again to check I'd done them right...

I am now wondering if there is another thread floating about here that suggests commenting out something that looks very similar, but ain't. There does seem to be quite a lot of confusion on this topic, and most threads seem to cover several related issues.

The specific problem I had, that was solved by commenting out the code you give above, was with a brand new clean install of Drupal 5.1: I did the install and could create users and receive the passwords by email, but was unable to get any further, as all pages gave an 'access denied' message and none of the admin menu links were showing.

I was installing on Pair Networks, in case anyone else out there is struggling with this - they give you a weird default URL of something along the lines of www123.pair.com/name456 .

As I was only using that one URL to get things up and running I had assumed that the stuff about multiple addresses / domains didn't apply to me, but it appears that this problem does still occur if you only have one URL, if the format of that URL is non-standard.

I experienced:
You don't have permission to access / on this server

I am now able to create the first user account after declaring my base URL in settings.php to my server address ccgi.***.com/drupal-5.1

Editing this seems to work because the source code does not account for particular URLs. I installed on a development machine with a url of the from xxx.xxxx.xxx.xxx:[PORT]. The regular expression did not make an acceptable domain name out of my mutant version of $_SERVER['HTTP_HOST']. I had noticed earlier that when I attempted to log in with an erroneous password, I would receive an error. When I used the correct password, I posted and was redirected to the "Access Denied" page. I checked the cookies and Drupal had set none, but obviously the password check completed successfully. I commented out the entire section. Apparently there was no reason to run ini_set(). If Drupal is not setting a cookie, but your login is behaving the way I described, try commenting out this section. If that doesn't work, I bet setting $domain = [your domain name]; will.

Instead of uncommenting the lines of code, activate them and change this:

$domain = '.'. preg_replace('`^www.`', '', $_SERVER['HTTP_HOST']);
==>
$domain = '.'. preg_replace('`^[a-z]+.`', '', $_SERVER['HTTP_HOST']);

This will get you the following lines of code:
/**
* We try to set the correct cookie domain. If you are experiencing problems
* try commenting out the code below or specifying the cookie domain by hand.
*/

if (isset($_SERVER['HTTP_HOST'])) {
  $domain = '.'. preg_replace('`^[a-z]+.`', '', $_SERVER['HTTP_HOST']);
  // Per RFC 2109, cookie domains must contain at least one dot other than the
  // first. For hosts such as 'localhost', we don't set a cookie domain.
  if (count(explode('.', $domain)) > 2) {
    ini_set('session.cookie_domain', $domain);
  }
}

See if it works, it does for me;)