Esellerate must have problems on some of their servers. I heard both Flightbeam and FSLabs, that use Esellerate too, are getting the same error.
If you try again, you should be prompted to do a Web activation, and that will work.
We already contacted Esellerate support, but they haven't replied to us yet, probably because of the weekend. The problem doesn't have anything to do with FSDT, of course, and it affects every other developer that use Esellerate, like FS Labs, FSDG, Flightbeam, etc.
On FS Labs forum, they suggest to use the Google DNS servers ( 8.8.8.8 and 8.8.4.4) instead of your default DNS in the network settings, while Esellerate figures out the problem.
