Undefined index: recent_poster_width
Settings.template.php
The guilty line is:
==>165: 'label' => $txt['recent_poster_width'],
It's a missing string inside one of the theme language files, but it's going to be related to a mod, as it's not something in the core SMF setup.
What should that read? You can then just add it into Themes/yourtheme/languages/Modifications.english.php:
$txt['recent_poster_width'] = 'what this string should say';
And, don't forget to clear your cache after you add it.
Admin -> Forum Maintenance -> Routine -> Clear Cache
Add this txt string to your_theme/Settings.english.php
$txt['recent_poster_width'] = 'Select width for recent poster window.';