Please try to deactivate wpForo for the sub-site you have this problem and activate it again. We don't recommend network activation, it's better to activate wpForo for each sub-site separately.

If the deactivate/activation doesn't help, please navigate to the sub site you have this problem, then go to Forums > Tools > Debug > Tables Tab and click on the button.

2019/04/23 20:46:31 19762#0: *306 FastCGI sent in stderr: "PHP message: PHP Warning: Declaration of c2c_ConfigureSMTP::options_page_description() should be compatible with C2C_Plugin_023::options_page_description($localized_heading_text = '') in /virtual/domain.com/htdocs/wp-content/plugins/configure-smtp/configure-smtp.php on line 47PHP message: WordPress database error Table 'sentryhoas.wp_12_wpforo_accesses' doesn't exist for query SELECT * FROM wp_12_wpforo_accesses made by require('wp-blog-header.php'), require_once('wp-load.php'), require_once('wp-config.php'), require_once('wp-settings.php'), do_action('init'), WP_Hook->do_action, WP_Hook->apply_filters, wpForo->init, wpForoPermissions->init, wpForoPermissions->get_accessesPHP message: WordPress database error Table 'sentryhoas.wp_12_wpforo_forums' doesn't exist for query SELECT * FROM `wp_12_wpforo_forums` ORDER BY `order` ASC made by require('wp-blog-header.php'), require_once('wp-load.php'), require_once('wp-config.php'), require_once('wp-settings.php'), do_action('init'), WP_Hook->do_action, WP_Hook->apply_filters, wpForo->init, wpForoMember->init_current_user, wpForoPermissions->get_forum_accesses_by_usergroup, wpForoForum->get_forumsPHP message: WordPress database error Table 'sentryhoas.wp_12_wpforo_usergroups' doesn't exist for query SELECT * FROM `wp_12_wpforo_usergroups` WHERE `groupid` = 4 made by require('wp-blog-header.php'), require_once('wp-load.php'), require_once('wp-config.php'), require_once('wp-settings.php'), do_action('init'), WP_Hook->do_action, WP_Hook->apply_filters, wpForo->init, wpForoMember->init_current_user, wpForoUsergroup->init_current, wpForoUsergroup->get_usergroupPHP message: WordPress database error Table 'sentryhoas.wp_12_wpforo_usergroups' doesn't exist for query SELECT * FROM `wp_12_wpforo_usergroups` WHERE `groupid` = 4 made by require('wp-blog-header.php'), require_once('wp-load.php'), require_once('wp-config.php'), require_once('wp-settings.php'), do_action('init'), WP_Hook->do_action, WP_Hook

I am using Version 1.6.1