This is the same issue I had earlier this week/late last week. Using Firefox on both Mac and Windows, now when I click "edit" on a block I'm logged out. I've emailed with Michael about this potentially being a sessions issue, but the browsers I use do allow for cookies.
This time I had successfully made several changes to several pages when the problem occurred. I'd updated the RSVP page and the form at the bottom of the page. For the latter I added some simple HTML code to the "Thank you" message.
At any rate, I'm now unable to edit in any browser. I don't mean to be pesky, just not sure what I'm doing that could be causing this.