Chrome 22 fixed it after a full cache & cookies reset. Strangely I had to clear those else I couldn't log in Jenkins.

Same here, running Jenkins 1.510 behind an Apache reverse proxy.

The <proxied URL>/$stapler/bound/6e8ac2e4-2fa3-4326-abff-bac3f3381a4d/render fails with error 404. Doing the same with <non-proxied URL>/$stapler/bound/6e8ac2e4-2fa3-4326-abff-bac3f3381a4d/render works.

Hopefully this information helps to diagnose the culprit.

Happens with:

client: Chrome 29, Firefox 17.0.8, or IE8
server: RHEL 6.4, Jenkins 1.529

Jenkins is running behind Apache reverse proxy using AJP protocol (mod_proxy_ajp).

I see this in Jenkins log file:
SEVERE: Error within request handler thread

winstone.WinstoneException: Unknown AJP packet type - 0
at winstone.ajp13.Ajp13IncomingPacket.parsePacket(Ajp13IncomingPacket.java:80)
at winstone.ajp13.Ajp13Listener.allocateRequestResponse(Ajp13Listener.java:184)
at winstone.RequestHandlerThread.run(RequestHandlerThread.java:69)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:439)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
at java.lang.Thread.run(Thread.java:662)

I cannot reproduce the issue on Firefox 17

I've never seen this either, despite reverse proxying.

Also, needs to be reproduced in a more recent Jenkins version, 1.535 being the absolute minimum, as that is when Jenkins.war started bundling Jetty instead of Winstone.