500 Error when go into edit post/page

Hi WPMUDEV,

Website URL is http://www.wshaguide.com/wp-admin(the WPMUDEV Dashboard is installed)

I try to edit a Post in Wordpress backend, however when I clicked edit post, it prompted me Http error 500. Same thing happened to the Page.
I'm not sure what is happening. This issue happened for few days already.

Here is the screenshot for the error message.

Kindly assist. Do ask me again if you require any further information.

Regards,
TS