A couple of weeks after upgrading to Jenkins 2.440.3, a "Bad Request 431 - reason: Request Header Fields Too Large" message began appearing in console logs, the script console, and other places.

This issue appears to have occurred numerous other times with most of them employing a workaround, like clearing browser cache. At least one of the issues indicated that the reason for the message is due to the session cookie getting appended to until it grows large enough to throw the error.

Here are some screenshots, with the first one on the script console page that appears to have redirected and the second from the console log of a running job (the console log stops outputting to the screen once the message appears):