Hi

when I want to backup my database from admin

http://www.domain.com/administration/db.php

and I select: Choose operation: Backup structure and content

than when i click Backup database I get this 500 Errror

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Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

i posted something  about this last year.      
if your db size is over 5 mb you won't be able to backup in admin section use phpmyadmin..