I was battling with this for a week. Spent many hours trying to work out what the problem was until today.
On the domain mapping options, this issue only occurs if I select the feature that lets users login across the network "Cross-domain autologin" useful, but it has caused hell.
Once a user is logged in, i.e. after they enter there password and login and press enter, on the odd occasion they are not redirected to their blog, but remain at wp-login.php where they see the white screen of death and then start to panic.
This is easy to test yourself, just active the plugin and then select "Cross-domain autologin", then navigate to yoursite.com/wp-login.php on a normal fresh install of wordpress you would see the login page again, but when "Cross-domain autologin", is activated you see the white screen. Please please please test your plugins before releasing them to us.