It looks like the plugin.rb file isn’t specifying the icons it needs. There should be lines like this near the top:
register_svg_icon "icon-name" if respond_to?(:register_svg_icon)
These weren’t needed in older versions of Discourse, but are now.
I’ve also found that some of the icon name change over time, I guess due to changes on the Font Awesome side, which has made some of my own plugin icons disappear until I found the new names for them.