Error when trying to login to backend

[scantron]

Hi everyone,

I have a problem with the website (yet again)
I cannot access the backend of my website, if i try i get following error message:

An error occurred while executing this script!

If you are a Contao user, contact the administrator of your website.
Open file system/logs/error.log and search for a corresponding entry.
Open file system/config/localconfig.php and add the following line:

$GLOBALS['TL_CONFIG']['displayErrors'] = true;

Read the Contao FAQs or visit the Contao support page.

To customize this notice, create a custom template file named templates/be_error.tpl.

When i add "$GLOBALS['TL_CONFIG']['displayErrors'] = true;" i get:

Warning: Cannot modify header information - headers already sent by (output started at /var/www/www.scantron.se/www/system/config/localconfig.php:65) in /var/www/www.scantron.se/www/system/libraries/System.php on line 520

#0 [internal function]: __error(2, 'Cannot modify h...', '/var/www/www.sc...', 520, Array)
#1 /var/www/www.scantron.se/www/system/libraries/System.php(520): setcookie('BE_USER_AUTH', 'd883840b3d8263c...', 1372149264, '/', '', false)
#2 /var/www/www.scantron.se/www/system/libraries/User.php(402): System->setCookie('BE_USER_AUTH', 'd883840b3d8263c...', 1372149264, '')
#3 /var/www/www.scantron.se/www/contao/index.php(84): User->logout()
#4 /var/www/www.scantron.se/www/contao/index.php(145): Index->__construct()
#5 {main}


Warning: Cannot modify header information - headers already sent by (output started at /var/www/www.scantron.se/www/system/config/localconfig.php:65) in /var/www/www.scantron.se/www/system/libraries/User.php on line 410

#0 [internal function]: __error(2, 'Cannot modify h...', '/var/www/www.sc...', 410, Array)
#1 /var/www/www.scantron.se/www/system/libraries/User.php(410): setcookie('PHPSESSID', '', 1372149264, '/')
#2 /var/www/www.scantron.se/www/contao/index.php(84): User->logout()
#3 /var/www/www.scantron.se/www/contao/index.php(145): Index->__construct()
#4 {main}


Warning: Cannot modify header information - headers already sent by (output started at /var/www/www.scantron.se/www/system/config/localconfig.php:65) in /var/www/www.scantron.se/www/system/libraries/Template.php on line 230

#0 [internal function]: __error(2, 'Cannot modify h...', '/var/www/www.sc...', 230, Array)
#1 /var/www/www.scantron.se/www/system/libraries/Template.php(230): header('Content-Type: t...')
#2 /var/www/www.scantron.se/www/system/modules/backend/BackendTemplate.php(143): Template->output()
#3 /var/www/www.scantron.se/www/contao/index.php(137): BackendTemplate->output()
#4 /var/www/www.scantron.se/www/contao/index.php(146): Index->run()
#5 {main}

Does anyone know how i can fix it?

[imported_Nina]

Which Contao version are you using? Did you validate the installation with the Contao check?