I thought I was going crazy but I have the exact same issue - I've even removed deluge, and triple checked the ports, restarted the server, docker, etc.
The problem is I can't even start the qbit container unless I map port 8080 > 8081, not that there were any conflicts but it's still an issue.
I'm assuming you're using either ubiquity tech, or Sab or some other business like I am.
EDIT: Okay, so apparently in the Unifi Controller - remapped port 8080 to something else but it's still holding that port as its own... I'm not an adept user so far but app