Transmission, the BitTorrent client installed by default in Ubuntu seems to have an issue with connecting to trackers using HTTPS at the moment.
After Transmission failed to connect to a tracker using HTTPS, I went searching and found this bug report.
The steps there are pretty simple and worked well for me:
- Close Transmission
- Download the .deb for updated libgnutls26 from here (bottom of page – choose your architecture then mirror)
dpkg -i (package name).deb
- Restart Transmission
UPDATE: It looks like this is now fixed, so running
sudo apt-get update sudo apt-get upgrade
should get it working again for you.