I had something very weird happen today. I was altering a template in Views, and after I clicked Update, I found that I could no longer load my web site (403 Forbidden error at the home page). After panicking and researching, I found the answer to be simple: I replaced my index.php at the root level of the WP install, with a new fresh copy. The site returned to normal.
However, from that point on, any changes I make in any Views template result in the red "Data Not Saved" error. I can't change anything in Views anymore.
I googled for the answer, and found that you recommended switching on WP Debug mode. I did that, and the results are available here:
http://daviddas.com/clients/wpviewsdebug.txt