child category page no post found 404 error

Hello,
I am getting this odd 404 error on the child category pages. I am using custom permalink /%category%/%postname%/ the parent category pages showes up but the child category pages shows “Nothing found 404 not found”

I have posts on those child categorys and they show up when I convert the permalink to the default. I have checked the .htaccess file and it has 666 as file permission.

I am using thesis theme 1.8

Help me solve this issue.

Thanks in advance for your opinion.