The easiest way to work around it is to stop bundles from merging. You can do this in the configuration registry by searching for "merge_with" and changing them all to "None".
I resolved it on my site by adding the following to registry.xml in my GenericSetup profile:
I made that change so relative URLs are not broken when bundles are merged, and it works with local URLs, but not with ++resources++ URLs (actually any URLs starting with "++" should be prefixed with "../../" and it should work).
Ok, I've used Daniel Marks' workaround and it worked.
First I've tried to change the values directly TTW in Configuration Registry but it didn't worked (error on submitting form)
The initial issue is difficult to understand. It seems to be related to plone.less first line: @fontsPrefix: '@{staticPath}/fonts/';
but well....
I had the same issue with the fonts fontello and glyphicons. They were all gone when installing plone.app.multilingual. I first tried to recompile the bundles in Development mode, but that did not change the situation. I could not update the Registry "merge_with" to None solution as Daniel discribed above and I have no idea how and where to put his solution with the registry.xml file and the GenericSetup profile...
So I solved it in a complete different way (and I admit not expected...) by using the portal_quick installer and did the following. I discovered that sometimes a simple reinstallation solves some corrupt things (my 5 cents):
Go to ZMI /...yoursite.../portal_quickinstaller/manage_installProductsForm
Maybe the reinstall of Static resource storage is not needed, but I simply started with that step and then the second plone.staticresources EXPERIMENTAL: Async Resource Loading.
After refreshing my browser (to prevent Caching issues) I got all Icons back