A crawler error means that Google AdSense crawler couuld not view your site's pages and index your site. By fixing these crawler errors, it will ensure that Google crawler can access your site and it will help to display more relevant ads.
1. First solution to fix this error is to grant the crawler access in your robots.txt file which is located at the domain level of your website. You can do this by adding two lines of text to the top of your robots.txt file:
This change will allow Google crawler to index the content of your site and provide you with the most relevant Google ads for that content. This change will not impact your Google search rankings. Google AdSense crawler will only attempt to crawl pages that have adsense ad code and are requesting ads.
2. In case of Page not found error Google Adsense Crawler is attempting to crawl a page that no longer exists. Ther are receiving a 404 error specifically for this page. So you have to make sure this URL is serving correctly.
3. Third issue is about the Content behind the login. Google deliver ads located behind a login. Sometimes account do not have crawler logins enabled and it prevents Google AdSense crawler from targeting the right content. So if your content and ads are login protected then you have to setup a crawler login. For login setup please visit http://support.google.com/adsense/bin/answer.py?hl=en&answer=161351.