Just installed 2.4.1, and am running into an error on a clean install. If I run a dev/build?flush=1, I get:

[User Warning] Cookie 'PastMember' can't be set. The site started outputting was content at line 76 in /Users/tybarho/Sites/sitesprocket24/sapphire/dev/DebugView.php
GET /dev/build

Line 33 in /Users/tybarho/Sites/sitesprocket24/sapphire/core/Cookie.php
Source

24          $path = ($path) ? $path : Director::baseURL();
25
26          // Versions of PHP prior to 5.2 do not support the $httpOnly value
27          if(version_compare(phpversion(), 5.2, '<')) {
28             setcookie($name, $value, $expiry, $path, $domain, $secure);
29          } else {
30             setcookie($name, $value, $expiry, $path, $domain, $secure, $httpOnly);
31          }
32       } else {
33          if(self::$report_errors) user_error("Cookie '$name' can't be set. The site started outputting was content at line $line in $file", E_USER_WARNING);
34       }
35    }
36    
37    /**
38     * Get a cookie variable
39     */

Trace

* Cookie 'PastMember' can't be set. The site started outputting was content at line 76 in /Users/tybarho/Sites/sitesprocket24/sapphire/dev/DebugView.php
Line 33 of Cookie.php
* Cookie::set(PastMember,1,90,,,,1)
Line 84 of Controller.php
* Controller->init()
Line 26 of DatabaseAdmin.php
* DatabaseAdmin->init()
Line 136 of Controller.php
* Controller->handleRequest(SS_HTTPRequest)
Line 134 of DevelopmentAdmin.php
* DevelopmentAdmin->build(SS_HTTPRequest)
Line 137 of RequestHandler.php
* RequestHandler->handleRequest(SS_HTTPRequest)
Line 147 of Controller.php
* Controller->handleRequest(SS_HTTPRequest)
Line 281 of Director.php
* Director::handleRequest(SS_HTTPRequest,Session)
Line 124 of Director.php
* Director::direct(/dev/build)
Line 127 of main.php

I can confirm this happens sometimes (I think most of the time on a dev/build after a fresh install).

Adding this to _config.php temporarily solves the problem:

Cookie::set_report_errors(false);

I created a new thread from this post: http://www.silverstripe.org/upgrading-silverstripe/show/290536