409: Conflict
Your request had an invalid session-id header.
To fix this, follow these steps:
When reading a response, get its X-Transmission-Session-Id header and remember it
Add the updated header to your outgoing requests
When you get this 409 error message, resend your request with the updated header
This requirement has been added to help prevent CSRF attacks.
X-Transmission-Session-Id: CBcYiodnQIHKYkhr9EceZOMW3ICgMSgt6j2FTCOXbcunA1tK
After hours of searching the solution a decided to use transmission from natty repository. So I did:
$ wget http://security.ubuntu.com/ubuntu/pool/main/t/transmission/transmission-common_2.13-0ubuntu8_all.deb
$ wget http://security.ubuntu.com/ubuntu/pool/universe/t/transmission/transmission-cli_2.13-0ubuntu8_amd64.deb
$ wget http://security.ubuntu.com/ubuntu/pool/universe/t/transmission/transmission-daemon_2.13-0ubuntu8_amd64.deb
$ wget http://security.ubuntu.com/ubuntu/pool/universe/t/transmission/transmission_2.13-0ubuntu8_all.deb
Then install the downloaded files:
$ sudo dpkg -i transmission*.deb
And repair broken dependencies:
$ sudo apt-get install -f
Now the transmission web GUI works again.