Unable to make permalink changes stick – changes back every time

Although I have no page at http://ganymedes-lgbt.nl/contacts, WP wont let me change the contacts page permalink from ‘http://ganymedes-lgbt.nl/contacts-2’ to ‘http://ganymedes-lgbt.nl/contacts’. I change it and click ‘save’, and everything seems fine until I refresh the page and I see the horrible truth; the edit was not made at all.

WP has the ability to write to .htaccess, if that makes a difference and the permalink structure is: /http:/ganymedes-lgbt.nl/%postname%
(I don’t know why there’s a forward slash before the http but WP insists that there be one)