First I'd try:
If that doens't work recheck your action-servlet.xml file to make sure something weird isn't going on (may be post part or all of it). Also, you can try turing on/up logging for the Spring classes, to see if you notice any suspicous messages that might point to a configuration problem.
<input type="button" onclick="location.href='editResource.html'"/>
Also, what browser do you use? If you aren't already using it, I highly recommend installing FireFox with the View Live HTTP Headers plugin (let's you see all the redirects, etc) as this can also help spot things like redirects, or form submissions that are pointed to incorrect URLs, etc.