[Quoth]

Buy ebook.
Wait
Go to Manage Content & Devices etc.
Select Download on ebook.
Pick the Kindle.
It says success, but attempting to save file gives

HTTP Status 400 – Bad Request
Type Exception Report

Message Request header is too large

Description The server cannot or will not process the request due to something that is perceived to be a client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing).

Exception

java.lang.IllegalArgumentException: Request header is too large
org.apache.coyote.http11.Http11InputBuffer.fill(Ht tp11InputBuffer.java:774)
org.apache.coyote.http11.Http11InputBuffer.parseRe questLine(Http11InputBuffer.java:442)
org.apache.coyote.http11.Http11Processor.service(H ttp11Processor.java:274)
org.apache.coyote.AbstractProcessorLight.process(A bstractProcessorLight.java:63)
org.apache.coyote.AbstractProtocol$ConnectionHandl er.process(AbstractProtocol.java:936)
org.apache.tomcat.util.net.NioEndpoint$SocketProce ssor.doRun(NioEndpoint.java:1791)
org.apache.tomcat.util.net.SocketProcessorBase.run (SocketProcessorBase.java:52)
software.amazon.disco.agent.concurrent.decorate.De coratedRunnable.run(DecoratedRunnable.java:114)
org.apache.tomcat.util.threads.ThreadPoolExecutor. runWorker(ThreadPoolExecutor.java:1190)
org.apache.tomcat.util.threads.ThreadPoolExecutor$ Worker.run(ThreadPoolExecutor.java:659)
org.apache.tomcat.util.threads.TaskThread$Wrapping Runnable.run(TaskThread.java:63)
software.amazon.disco.agent.concurrent.decorate.De coratedRunnable.run(DecoratedRunnable.java:114)
java.lang.Thread.run(Thread.java:750)
Note The full stack trace of the root cause is available in the server logs.

Maybe someone broke something? I see a massive gratuitous change just now to my Kindle Wish List thing. The notes are now in a tiny font and the layout looks designed for a phone.
A web browser reports window size. What idiots can't do "responsive" design?

Tried several browsers and with script blocking off.