For those guys who may have the same problem with me.....
I just tried to install this extension to make realname append to user ID on top of page.
But I didn't notice that all my page titles (<h1>) are gone since I didn't give arguments for $wgRealnamesReplacements
One day, I found this problem and spend couple hours to solve this strange problem.
Finally, I realized I should set $wgRealnamesReplacements['title'] = false to prevent from title missing.
It's my misunderstanding....Just hope no one will do stupid thing like me.