The last few days, people have frequently been getting logged out on Beta Cluster with "Sorry! We could not process your edit due to a loss of session data." messages when an action is taken (e.g. after edit, or saving preferences).
This is similar to T151770: Frequent loss of session data in Firefox (since around 2016-11-28), but the current task reportedly happens in Safari and Chrome (as well as Firefox).
Someone could try posting their headers (see https://phabricator.wikimedia.org/T151770#2875829 ) from a case where they hit the bug. If you do so, be sure to log out after capturing the headers and before posting them.