If you run phpMyAdmin in combination with PHP's APC caching module, you might find your phpMyAdmin returning blank white pages instead of the actual management interface. In your errors logs, you can see messages like this.
PHP Fatal error: Class 'PMA_Error_Handler' not found in /path/to/phpMyAdmin/libraries/common.inc.php on line 58
PHP Fatal error: Call to undefined function PMA_getenv() in /path/to/phpMyAdmin/libraries/common.inc.php on line 143
The solution is to exclude phpMyAdmin from the APC cache, which you can do with a following parameter in your Virtual Host configuration.
php_admin_value apc.enabled 0
Update the path above to the correct directory where phpMyAdmin resides, and restart your Apache to activate the config.
If the above does not fix it, you can also try to clear your PHP sessions, which are known to cause issues with phpMyAdmin. Especially if your php.ini is lacking explicit "session.save_path" declarations (by default they point to /tmp).
session.save_path = "/tmp"