Error: The server could not perform the XSL transformation because an XSLT processor for PHP could not be found.

Reason: The extension=php_xslt.dll is commented out in your php.ini file for your site

Resolution
1. Login to your site via FTP or use the PLESK File Manager Utility.
2. Change directories to the httpdocs directory
3. If using FTP, download the php.ini file, locate the line ;extension=php_xslt.dll and remove the ';' character
4. Save the php.ini file and re-upload the php.ini file to your httpdocs directory

 Be sure to also read the following article regarding uploading a custom php.ini file:

Avoiding PHP problems: Uploading php.ini as binary or ASCII