Transmission webui problem - Conflict 409
Solution 1:
What is the URL you're trying to use?
Don't forget the trailing slash http://host:9091/transmission/web/ (it makes all the difference)
Solution 2:
Try adding #files to the end of the address. e.g /web/#files.
It worked for me.
I was even able to take the #files off it once I had logged in and it still worked for some strange reason?
Solution 3:
I just solved this issue enabling the DMZ server in the router, pointing to the local IP of the raspberry.