This has just happened to me. I noticed that plain text saved OK, but any attempt to markup caused the save to fail.
May be more than one thing that causes this, but as I noticed that my dashboard was screwed up also, I found the answer at http://wordpress.org/support/topic/117847
I followed the suggestion of whoami (thanks, if you are reading this) and added the .htaccess file to wp-admin.
Solved both problems immediately
