Hi Roger,
The blank screen means you're getting a php error but you have display_error set off in php.ini or in the boot path.
You may have an error in your login path, your session table may be corrupted, you might be running out of memory on your server, or the server may be having trouble setting a session cookie on your browser. These are common problems.

Try adding
error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);

after the boot code in index.php
Also check your apache logs watchdog table to see if you're getting errors.
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-Don Pickerel-
Fane Software