SVG image processing doesn't work with ADFS and proxy

[fschrempf]

I'm using the template with DokuWiki behind a nginx reverse proxy and with the ADFS plugin to authenticate users via SAML. When a user is not logged in everything works as expected. When a user is logged though, the processing of SVGs via svg.php fails. Instead of returning the image, svg.php redirects with status 302 to the auth endpoint of the SAML IdP. So far I have no idea why this happens.