Each time Jenkins (re)starts, its session-cookie name changes (ie JSESSIONID.some_random_string).
After a while, the browser have a bunch of session cookies, each one having a different name, causing the "Cookie" request header to be very long. The server returns a HTTP 413 response and a blank page. The user must clean his cookies in order to access Jenkins again.
Workaround: Since Jenkins 2.66 there are custom options for managing Jetty session IDs: https://github.com/jenkinsci/extras-executable-war/#jetty-session-ids