Ok, I start to get the big picture. It seems that the problem is limited to Windows Phone 7 devices. Windows Phone 8, Androids, Kindles are fine. of course, you may have different issues on these, but they are something else and don’t affect all of the downloads all of the time.
Please keep letting me know whether you cannot download any community content (worlds or block textures), even though your phone has a working internet connection. Please try downloading several items before posting a comment, preferably from the top of the list. If you get the same error “cannot open as zip file” for all worlds it’s this problem.
I have traced the issue to the fact that several days ago Dropbox servers have silently changed the way they are handling public links. Instead of giving you the resource, you are getting a HTTP 302 response (redirection), and have to download from another address. Network stacks on Android and Windows Phone 8 seem to handle this redirection OK, as it’s supposed to be. But on Windows Phone 7 the stack is flaky and redirections don’t work. Instead of the resource (i.e. the world or blocks texture data), the game gets some HTML with human readable text. It then tries to unpack it (thinking it’s a scworld file), but of course this fails. That’s why you’re seeing these errors.
If this turns out true, I will have to make a special workaround for WP7 in 1.21.