Hi there -- I hopped into your account to see if I could isolate the internal server error. I got that error when working in your latest draft and could see that your template was breaking a bit...the right edge was being pushed out by something in your mailing. That internal server error most often points to something in the mailing content that isn't compatible.
I believe you had some hidden formatting on the text in the right column that was cluttering up the HTML behind that text. I rebuilt your mailing in the EMMA COPY and pasted that text in *without styles*. That's the key to leaving any extra formatting behind. I was able to send that mailing as a test to myself without receiving the error.