I was running into a situation where I could view my site with no problem, but when I tried to login I was getting a blank page. I could get to the logon form but when I hit the login button I got a 200 back from the server, but no content.
After some digging around with directory permissions and log files to no avail I moved everything from my plugins directory into a temporary directory and refreshed the plugins directory with the original files. After that I was able to logon so one by one I moved my plugins back and tried to logout/logon. It turns out I'd goofed a hack in my cg-amazon.php plugin.