I have custom WordPress Roles, such as, Leadership (Read), which is duplicated from Subscriber. This particular role is a supplemental role which has access to certain things. When this role is either the Primary or a 2ndary role to a Primary role of Subscriber, the user cannot log in. They will receive the white page. As soon as I turn the WordPress role back to Subscriber (and remove any secondary roles), it will work. This recently became an issue, but I do not know why.
This is also affecting the User Switching plugin too, so I can not even switch users anymore. Once the 2ndary roles are removed, it starts working immediately.