In our experience it happens from time to time that one gets a "white screen of death" - short WSOD - after the login with the admin account. Most frequently this happens after the move of a Drupal website to a new host. This is of course very odd and can lead to a very time consuming troubleshooting. Unfortunately we did not find out the precise causes and we couldn't find clear answers either. But processing the following set of measures always helps us to resolve the problem.
Furthermore there is a workaround for the login: Create an editor with almost full rights - you have to uncheck "flush caches" and "Use the administration pages and help" - before the move, allowing you to login and perform almost all the operations.