WP always point me to 404 page when I save something

Hi guys,

Since yesterday, I found it is difficult to write the settings of posts in the wordpress backend. Every time when I click on something to let the back end save the data to DB it will point me to the 404 page. But occasionally, it can function well. And sometime it returned me the 404 page, however the data is actually wrote into the DB. The front is more stable. But some visitors informed me that they saw internal ERROR 500. After refresh, it works normally.

Before the problem, I installed some plugins like w3-totle-cache and head cleaner to optimize the site. Now I have disabled them.

My site is hosted on dreamhost by the way.

Anybody knows where the problem is? Thanks!