Fatal Error When Plugin Activated

I noticed the plugin not working recently and saw it got deactivated. When trying to activate the plugin I received this error:

Fatal error: Uncaught Error: Call to a member function get_author_permastruct() on null in /var/www/wp-includes/author-template.php:347 Stack trace: #0 /var/www/wp-content/mu-plugins/gd-system-plugin/includes/class-cache.php(253): get_author_posts_url(0) #1 /var/www/wp-includes/class-wp-hook.php(287): WPaaS\Cache->do_purge(2219, Object(WP_Post)) #2 /var/www/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(”, Array) #3 /var/www/wp-includes/plugin.php(484): WP_Hook->do_action(Array) #4 /var/www/wp-includes/post.php(6955): do_action(‘clean_post_cach…’, 2219, Object(WP_Post)) #5 /var/www/wp-includes/post.php(3108): clean_post_cache(Object(WP_Post)) #6 /var/www/wp-content/plugins/contact-form-7-paypal-add-on-pro/includes/admin/files.php(24): wp_delete_post(2219, true) #7 /var/www/wp-content/plugins/contact-form-7-paypal-add-on-pro/paypal.php(136): include_once(‘/var/www/wp-con…’) #8 /var/www/wp-settings.php(388): include_once(‘/var/www/wp-con…’) #9 /var/www/wp-config.php(99): require_once(‘/var/www/wp-set…’) in /var/www/wp-includes/author-template.php on line 347

Anyone else get this?

I’m on version 2.6.15 since I was encountering the “Price can’t be 0” error on version 2.7.