Mira a ver si lo puedes solucionar con esto:
Why do I see a white screen when importing demo content?
If you get a white screen or some other error when trying to import our demo content, this probably happens because of the maximum execution time limit. You need to increase the maximum execution time (upload time) setting of your web server. The default maximum execution time on web servers is 30 seconds. Please increase it to 120 seconds. Possible ways of achieving this are:
By Wp-config.php changes - set_time_limit(120);
In htaccess - php_value max_execution_time 120;
In php.ini file - max_execution_time = 120
Ask your hosting provider to take care of this for you.