Try this,
Do not launch the sub-report from the main report, rather open the sub report by itself from infoview and try to perform the action. I assume something may be happening in the Opendoc. if you can narrow it down to opendoc, then you can work on how you need to build the opendoc URL.
Just need to do the natural troubleshooting.
try to open the sub report in Webi righ client and publish it back, see if it fixes anything. I googled and found something about post back issues and that seems to be a problem. There are a few links on good when i searched using "MochiKit.Async.XMLHttpRequestError", they should help you or at the least give you a direction.