My first guess would be that you only upgraded the .bin file and didn't bring in the entire .tar file which also includes the http file you'd need. Or, maybe you haven't reloaded the switch yet, upgrades can put the http files into different locations than the original ones and therefore can't find the need files.

another thought, is the new version one that you can only access the web interface by downloading and installing the Cisco Network Assistant and accessing the web interface through there.