White screen after logging in

I just installed and test this plugin intending to solve the problem of my login/logout widget on the sidebar. Because of caching, the user status is not reflected in the sidebar.
I follow the instruction to drag the Ajax Widget to the main sidebar, then added the login/logout shortcode in the Ajax Widget. When I refresh a page, it looked fine. But when I tried to login, I got a white screen. I have to press the Back button on the browser to get back my screen.
How can this be solved?
In addition, the loading of the Ajax Widget is quite slow.