Hi, I've had the same issue thatthis post describes on a couple of my client's sites*, although don't have the 'Search Everything' plugin installed as they did.
I can log in fine as an administrator, but when the client tries to log in it results in an unexplained blank screen. (Debugging shows a 500 error)
After some elimination I found that disabling 'White Label CMS' and then re-enabling it fixed the issue, so that seems to point to the issue involving this plugin. Also it'd make sense as the plugin concerns itself with customising the wp-login page.
*I've had the issue with the following sites: