Are there any situations in which drones may legaly fly higher than 400ft?
As they say with any regulation, there’s a waiver for everything.  If you do wish to operate your sUAS at or above 400’ AGL, you are advised to contact the local FSDO in your area.  Protocol varies from area to area, but you will be required to file an request with local ATC with the dates, times and areas you intend to operate in so NOTAMS can be issued.