[shnfoobar]

I think the point here is this message.

"Tracker sending invalid data: <NULL>"

That is... the tracker is the one sending invalid data. Not us, our torrent client or our proxy settings.
This is something that only TTD Admin can address.
It is entirely possible that the cause is within this response from 2008:

http://www.thetradersden.org/forums/...ad.php?t=58132
"You are seeding your maximum number of ten torrents currently. All others will show up with Invalid Tracker status. I suggest you only choose to seed a couple of torrents that need to be seeded. (Several you are 'seeding' have 0 leechers and therefore don't need to be seeded.)"

Now, we know that we're not exceeding our maximum because we're closing down everything unneccessary to try to get just 1 working.
However, this problem was encountered elsewhere and the code monkeys did succeed in finding a rogue bit of code that was causing a limited number of users to have this problem because the tracker was mis-counting their activity.
Good luck !