I'm running 11.0.4 and can't seem to get the file path field to reference a css file in a static resource zip file. When I search the file path it doesn't give me any results to select and so I can't save.
I encountered this as well. My workaround was to move the page to a 9.5 org, edit it there, then move it back. If you have the same static resource in both orgs it should work, just don't touch it once it comes back to Millau.