Since the arena is always empty, I thought I would try to raid.
The message says; you won, when no one is there.
& when someone is there it is an error message.
The error message comes when you find someone and that person gets raided by another player before you do.
I have not been able to raid since the update. All I ever get is you have defeated all opponants. 
But yet I get raided once about every 10 hours.
It seems to matter now which residence level you have - the people you want to raid need to be close to your own residence level.
Well, I wish the peeps that keep raiding me would go to sleep when I am awake trying to raid. hehe