Dear Vialla,
Thank you for those php warnings. Actually, none of them come from the downloads plugin, and none of them should cause a blank screen.
Are those warnings/notices the only thing that is printed in the web browser when you enable development error reporting for Joomla?
If you want, I can take a look at your site (you can send a temporary admin account to office@open-tools.net).
If you are not comfortable with given access to a complete stranger, can you at least give me the the full URL of a download, so I can see the problem myself (and also check what happens if the order password is not correct)?
Best regards,
Reinhold