for the record, the related dojo bug report: http://trac.dojotoolkit.org/ticket/10985
conclusion: browsers behave weird and different, but simply returning an empty xml answer with a reverted iolib does not provoke the erratic behavior i see with iolib head.
i've done a bit more experimenting and i have one more data point: firebug in ff shows that the http request is running indefinitely long (several minutes later the progress indicator still spinning), even though the server log shows that handling the request have finished (with a call to close in uwp and no sign or error). maybe the browser does not receive the socket closed notification?
the console in chrome doesn't even show the xhr request being made, i guess chrome only logs it after it has finished.
hth,