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I believe that the problem is not with the backup and restoration but something different. A white page or a page with a 500 Internal Server Error is, in fact, either a .htaccess issue to a PHP fatal error in disguise.

First, let's see if it is a .htaccess issue. Try renaming the .htaccess file in your site's root to htaccess.bak (try both the main site's .htaccess AND the restored site's .htaccess please). If there is a .htaccess file in the site's administrator directory, try renaming it as well. If that solves the problem, the issue was with a directive in your .htaccess file. We'd like to recommend you to try removing directives from your .htaccess until you find the one which causes the problem.

If that doesn't help, the error you are receiving is in fact a PHP error in disguise. First, check your server's error logs (not the access logs) immediately after visiting the page which throws the error. There should be an exact description of the PHP fatal error which occurred. Sometimes you can find the error messages in files called error_log or error.log inside the site's root and/or administrator directories. If unsure about the error log location, please consult your host. Most likely the error logs are available in your site's cPanel, Plesk control panel or similar hosting account management facility.

If your host does not give you access to the error logs and you have access to the Joomla! administrator area, please log in to your site's back-end, go to Global Configuration, click on the Server tab and set the Error Reporting to Maximum (Joomla! 1.5) or Development (Joomla! 2.x and later). Try visiting the problem page again.

If you still get a blank page, edit your configuration.php file and put the following code right after the final closing curly brace ( this is what a curly brace looks like --> } ) but before the closing PHP tag (it looks like ?> that is a question mark and a greater-than sign):

ini_set( 'display_errors', true );
error_reporting( E_ALL ); 

Try visiting the problem page again.

If you still get a white page, please remote the two lines from your configuration.php file. Edit the .htaccess file in your site's root (not the main site, I'm talking about the restored site). If you don't have a file named .htaccess create a new one. Beware that htaccess.txt is a DIFFERENT FILE and will NOT work! Add the following to the end of the file:

php_flag display_errors On
php_value error_reporting 32767

and retry loading the problem page.

If you still get a white page, remove the two lines from your .htaccess file. Now, create a file called php.ini with the following content:

display_errors=on
error_reporting=E_ALL

and upload it into your site's root and your site's administrator directory. Retry loading the problem page.

IMPORTANT: Sometimes the error won't show. Edit your configuration.php and find the line starting with public $error_reporting and set it to:

public $error_reporting = 'development';

This instructs Joomla! to enable maximum error, warnings and notices verbosity. In 99.9% of cases it will result in the error messages being shown on the browser.

Please note that if you can not understand what the PHP error message means, just copy and paste it here verbatim so that we can take a look and point you to the right direction.

This doesn't shed any light at all, I'm afraid. Can you please set your site's error reporting to "Development" and set "Debug Site" to On? If logging in to your site throws a blank page please try replacing the index.php in your browser's address bar with index.php?option=com_config which will take you directly to the Global Configuration page where you can change these settings. Then retry accessing your site; it should spit out much more PHP messages which should tell us what is going on.

That's very interesting... because that line with that content doesn't exist in the latest (and the last to latest) versions of our software. This means two things:
1. For some reason Akeeba Backup and Admin Tools are not updated. Please try installing both components (Akeeba Backup Pro 3.10.1 –just released– and Admin Tools Pro 2.6.0) TWICE in a row, without uninstalling them first. This will convince Joomla! to really write the updated files on your server. This is something you have to do on your main site. Then take a new backup and retry the restoration.
2. If I'm guessing correctly, you seem to be trying to restore a backup on top of an older site. If this is the case, wipe the entire osobackup folder and retry the restoration.