Hi ismgr
Do you have SW contract for the controller?
If yes, don't try to download from My Networking portal. Instead go to http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c02665350 for instructions. The page contains a link to Software Support online -portal which you use for devices under SW contract. Using wrong portal is the most common reason why FW download fails with this error.
As was mentioned FW 6.0 is currently under Early Availability state. EA releases are intended for testing out new features before deploying to production network. Using it in a production network should be done with caution, as all issues which are fixed in 5.7.2 and above are at the moment known issues with 6.0.
If you currently have a stable wireless, don't go to 6.0. If your current isn't working properly, call support and we'll see what we can do about it.