Joomla! 2.5.5 has known issues. They are not caused by using Admin Tools to upgrade your site and we can not offer support for them. Please read the rest of this article for more information.
Joomla! 2.5.5 has a known issue which is affecting third party extensions (but not Akeeba extensions!). This was a schduled change of a deprecated API. Unfortunately, many developers did not follow up on the change, causing their extensions to throw fatal errors, blank pages or 500 Internal Server Error when used on your sites. We regret to inform you that we can not debug third party extensions running on Joomla! 2.5.5 as that's outside the scope of the support services we are offering. You will have to wait for a new Joomla! release which addresses this issue (there is already a patch available) or consult with the developer of the misbehaving extension. The following instructions will help you find out which extension is causing the issue.
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. 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.
If you still get a white page, delete the php.ini files your created and choose a different host. If your host doesn't allow you to debug any PHP-related issues there is no point paying them.
The PHP error message you receive should point you to the extension causing the issue. Unfortunately, that's all the help we can offer for this issue which is not related to our software.