Search Crawl Error - Access is denied. Verify that either the Default Content Access Account has access to this repository, or add a crawl rule to crawl this repository.....

Problem

After Windows updates were applied to the SharePoint Server, the search service failed to crawl the content. 

Error

"Access is denied.  Verify that either the Default Content Access Account has access to this repository, or add a crawl rule to crawl this repository. If the repository being crawled is a SharePoint repository, verify that the account you are using has “Full Read” permissions on the SharePoint Web Application being crawled."

Cause
The windows update actually enabled the Loopbackcheck leading to crawl failure. The other related problem you will often observe is that you cannot login as another user if you have opened Central Admin on the server itself.

Resolution

There are two possible solutions listed at the Microsoft KB article.

Solution 1: Specify host names (Preferred method if NTLM authentication is desired)
Method 2: Disable the loopback check (less-recommended method)

For detail steps follow this link  to the KB article.