I've been suffering through this for a while now, with it eventually leading to all 31 of my active torrents no long seeding. Downloading was fine, just not the upload. I looked at the actual tracker URL, and it was correct, wasn't the public one or whatnot. Eventually was able to "fix" it by setting them up to force seed. This isn't a perfect solution, but seems to be working for now. Change your settings from "Beginner" to "Advanced" and then right click the affected torrents, select "Force Start."
Unless someone has the real, technical solution such as Java needs to be updated, privacy settings got too strict somehow, ISP started mucking things up, whatever else. Then by all means please share.