can't upgrade to 5.5.1

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Hi,

I run 5.5.0 and this runs fine. Now i want to upgrade to 5.5.1 via de update section of te 5.5.0.
When i press the download button i get the error:
The directory /var/www/vhosts/duikvereniging-submarine.nl/subdomains/submarine2012/httpdocs/updates must be writable by the web server.
The admin from the hosting site changed the properties to 775 and 777 recursively to all folders.
But it still does not work.
I am logged in as the admin.

Is there an alternative to do this?