Feed import breaks due to ajax error
By using xdebug and vdebug (plugin for vim), I am able to step into the code and ping point the issue.

In https://api.drupal.org/api/drupal/includes!common.inc/function/drupal_cron_run/7

I put a watchdog: watchdog('bug_cron', '&lt;pre&gt;'. print_r($module, true). '&lt;/pre&gt;'); inside.

// Iterate through the modules calling their cron handlers (if any):
foreach (module_implements('cron') as $module) {
      // Do not let an exception thrown by one module disturb another.
      try {
        // NOTE: put a watchdog and look at your log message. 
        // You should able to see which module causes issues.
        watchdog('bug_cron', '&lt;pre&gt;'. print_r($module, true). '&lt;/pre&gt;');


        module_invoke($module, 'cron');
      }
      catch (Exception $e) {
        watchdog_exception('cron', $e);
      }
    }


It is the search module causes issues. Disable it, then uninstall the setting via the drupal menu GUI, then it is fixed, but I am not sure why search module causes issues.

Updated at 2015-05-26

It turns out my module uses hook_url_inbound_alter + drupal_goto caused issues. It is not easy to debug, because there are couple of side effects.