I'm not sure if this is a Views bug or a "security patch for 8.5.1" bug.
How to reproduce
1. Create a Drupal 8.5.0 site.
2. Create a view and set the path to user/% to override the user profiles.
3. Upgrade the site to 8.5.1.
4. /user/login and /user/logout will now show the 404 page. Using Drush to log in and removing or changing the path of the view set to "user/%" will allow login and logout to work normally.
I have always used "user/%" in a view to override the user profile display, but since 8.5.1 it does not work.
Reference info
Additional details can be found in this forum support thread and on Drupal Answers.