During a Jira upgrade there is a maintenance task of clearing out the user session tokens. These user session tokens are created when someone clicks on "Remember me" when they are logging in. A screenshot of the administrative panel where this is done is below:

When these tokens are cleared, people who are used to not having to sign in will have to sign in and click on "Remember me" to create a new session token. Furthermore, if someone leaves an issue open the day before the upgrade and comes back they will find that when they try to work on the issue it will show a blank white screen until they log back in. The screen looks like this:

[Screenshot pending reproduction of the issue]

In summary, after a Jira upgrade all users will have their "Remember me" tokens cleared and will have to log back in. There may be some service desk tickets that come in from people who think that Jira is malfunctioning when logging back in will resolve their issue.