Crawling component stopped working: Access is denied

A user reported that he could not do any search from a SharePoint site. I checked the log, and found the error message below.

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

It worked before, and other web applications all work well. Anyway, I doubled checked:

1. The rights of Default Content Access Account over the web application ("Full read");
2. DisableLoopbackCheck problem;
3. "Basic authentication" issue;
4. Change content access account;

When "start full crawl", the same error is always there.

I think something crashed. So I added one more "Crawl Component", and then deleted the old one.

[update 2013-09-24]
Yeah, the problem disappeared.

Unfortunately, although the crawling went well, the query didn't.

I checked the whole web application features through FeatureAdmin, nothing wrong there.

There is only one content database, so I think maybe something wrong with the content database. Test-SPContentDatabase gave me the message below:

Test-SPContentDatabase : Object reference not set to an instance of an object. At line:1 char:23
+ Test-SPContentDatabase <<<<  -Identity 4b146bda-9a6e-4ef7-8ed1-eb073ca0bccb
    + CategoryInfo          : InvalidData: (Microsoft.Share...ContentDatabase:    SPCmdletCheckContentDatabase) [Test-SPContentDatabase], NullReferenceException
    + FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletCheckContentDatabase

I spent a while, and didn't know how to fix it. It seems something wrong with the schema, but don't know what is wrong. So I backed up all site collections of that web application, then rebuilt the web application and did another full crawl.

Guess what? The problem was still there!

So I did an index reset, then, ok, the problem was still there!!

After a few days struggling, almost by accident, I rebuilt the web application, then did an index reset immediately.

That fixed the problem.

I know it doesn't make sense, but anyway, it works now.

( The SharePoint 2010 farm has got SP2 and CU201308 )

Reference link: http://wingleungchan.blogspot.com.au/2011/11/access-is-denied-when-crawling-despite.html