Upgrade error

Hi

I have updated thru all versions to 2.5 and it still crashes the site. I’m stuck in v2.2.2

This is the current error
Fatal error: Uncaught Error: Call to undefined function wp_get_current_user() in /home/customer/www/*****.com.au/public_html/wp-includes/capabilities.php:693 Stack trace: #0 /home/customer/www/*****.com.au/public_html/wp-content/plugins/ultimate-member/includes/core/class-access.php(433): current_user_can('administrator') #1 /home/customer/www/*****.com.au/public_html/wp-includes/class-wp-hook.php(307): um\core\Access->exclude_posts(Object(WP_Query)) #2 /home/customer/www/*****.com.au/public_html/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters('', Array) #3 /home/customer/www/*****.com.au/public_html/wp-includes/plugin.php(524): WP_Hook->do_action(Array) #4 /home/customer/www/*****.com.au/public_html/wp-includes/class-wp-query.php(1805): do_action_ref_array('pre_get_posts', Array) #5 /home/customer/www/*****.com.au/public_html/wp-includes/class-wp-query.php(3586): WP_Query->get_posts() #6 /home/customer/www/*****.com.au/public_html/wp-includes/post.php(2410): WP_Query->query(Array) #7 /home/customer/www in /home/customer/www/*****.com.au/public_html/wp-includes/capabilities.php on line 693