That's normally caused by a broken link or a bad personalization tag, among other code-breaking issues that could arise.
I popped into your account and into the "A+D Newsletter"... which seemed to be the one you were working on. It looks like the first image block there had the beginning of a link, but it wasn't completed. (It's what the proofing error was pointing to as well)
Once I completed the link, I was able to send a test.
If that's not the mailing you're looking at, let me know the name of the mailing and I'll be happy to pop in and take a look.