@Tunghsiao Liu: The problem may be fixed in the latest beta of mod_pagespeed, which your site appears to be running already. Can you try adding:
ModPagespeed on
ModPagespeedAllow */wp-admin/*
to either your pagespeed.conf or .htaccess in your wordpress root directory and see if the wp-admin pages:
1) do have pagespeed applied to them (look for .pagespeed. resources in the source)
2) work correctly?
Thanks!
Josh
