I am using Excel web access web part to display Excel reports in Sharepoint 2010. It's working fine with IE9. When I am loading the report in IE 10, it's throwing the error "Unable to get property 'PermissionFlagsHash' of undefined or null reference " in ewamoss.js file.
Currently, IE 10 (release preview) is not supported in SP 2010.
In the meantime, try this. Press F12, to bring up the developer tools. In the Script tab, debug ewamoss.js file to see why you are getting a null reference.