Ubuntu 12.04 64bit currently suffers from [https://bugs.launchpad.net/ubuntu/+source/gnutls26/+bug/937537 this bug] in libgnutls26 which prevents Transmission communicating with SSL trackers. In order to fix this run the following commands.

sudo dpkg -i libgnutls26_2.12.20-1_amd64.deb

For a list of available mirrors see [http://packages.debian.org/wheezy/amd64/libgnutls26/download here]