Luhmann, I think one of the sites listed above uses Bad Behavior, the remainder either haven't started yet or aren't blogs, forums, wikis or other CMS software.
The short 412 error you are seeing is an error returned by TextDrive-hosted sites. It has nothing to do with Bad Behavior, which although it also uses the 412 error, is far more verbose. You would know if you saw a Bad Behavior error message. Among other things it goes on for more than a page, and also gives you a contact email address.
TextDrive uses a somewhat different method of blocking spammers, which relies mostly on User-Agent. This is probably why you were able to get through using a proxy server, as many of them change the User-Agent. If there's anything funny in your User-Agent, try setting it back to the default. If you still can't get through, contact TextDrive.
I would recommend against using open proxy servers for your regular browsing, though, as many sites will prevent you from posting, or ban you outright, if you access them via an open proxy, because nearly all spam comes from open proxies. I've got a project in progress dealing with the open proxy problem where you can find some more info.