Logfile (Firebug output) and Error codes from the response 500-error site
When I try this on my talk-page it works but the response is a server-error instead of a normal one. I assume the problem arises because it's a redirect.
But nevertheless there are other users complaining getting server error 502 when attempting to add text to a discussion page that is not a redirect:
A description/ log of the error I got is attached.