Fatal error, class 'Walker' not found

Hi,

I believe this fatal error has to do with the theme being used.
In my apache error log is the following:

[Thu Jul 04 13:18:17 2013] [error] [client 46.38.209.228] PHP Fatal error:  Class 'Walker' not found in /var/www/gemeentehollandskroon/public_html/wp-admin/includes/template.php on line 23, referer: http://hollands-kroon.nl/wp-admin/network/update.php?action=update-selected&plugins=jetpack%2Fjetpack.php%2Ctradetracker-store%2FTradetracker-Store.php%2Cwordpress-chat%2Fwordpress-chat.php
[Thu Jul 04 13:18:17 2013] [error] [client 46.38.209.228] PHP Fatal error:  Class 'Walker' not found in /var/www/gemeentehollandskroon/public_html/wp-admin/includes/template.php on line 23, referer: http://hollands-kroon.nl/wp-admin/network/update-core.php?action=do-plugin-upgrade
[Tue Jul 16 08:54:28 2013] [error] [client 46.38.209.228] PHP Fatal error:  Class 'Walker' not found in /var/www/gemeentehollandskroon/public_html/wp-admin/includes/template.php on line 23, referer: http://hollands-kroon.nl/wp-admin/network/update.php?action=update-selected&plugins=bulletproof-security%2Fbulletproof-security.php%2Cwordpress-chat%2Fwordpress-chat.php
[Tue Jul 16 08:54:33 2013] [error] [client 46.38.209.228] PHP Fatal error:  Class 'Walker' not found in /var/www/gemeentehollandskroon/public_html/wp-admin/includes/template.php on line 23, referer: http://hollands-kroon.nl/wp-admin/network/update.php?action=update-selected&plugins=bulletproof-security%2Fbulletproof-security.php%2Cwordpress-chat%2Fwordpress-chat.php
[Tue Jul 16 08:54:38 2013] [error] [client 46.38.209.228] PHP Fatal error:  Class 'Walker' not found in /var/www/gemeentehollandskroon/public_html/wp-admin/includes/template.php on line 23, referer: http://hollands-kroon.nl/wp-admin/network/update.php?action=update-selected&plugins=bulletproof-security%2Fbulletproof-security.php%2Cwordpress-chat%2Fwordpress-chat.php
[Mon Jul 22 10:11:04 2013] [error] [client 46.38.209.228] PHP Fatal error:  Class 'Walker' not found in /var/www/gemeentehollandskroon/public_html/wp-admin/includes/template.php on line 23, referer: http://hollands-kroon.nl/wp-admin/network/update.php?action=update-selected&plugins=wordpress-chat%2Fwordpress-chat.php

Can this fatal error be fixed?

Gr,
Age