I switch very often of iconset. Some iconsets do not use the default names for their files.
If the iconset is switched, you have to edit the missing icons and replace them for another one (if you want to mantain the aspect on all icons).
This php script helps you to manage your iconsets. You can find which are the missing iconsets from the default one, and rename or copy other icons so that the names are matched.
Make sure your icon directories can be read and written by the same user who executes the apache server.