After filling the key and secret directly into the database I got a little further: I get the authorizing link and get to give the permissions on Flattr’s side, but then I get a blank page (…/callback.php?oauth_token=…), so the permissions are never actually granted.

I should mention this is one of multiple blogs running under one WP installation (a MU-like installation, if you like). I’m the admin of my site so I’ve tried things as both the blog admin and the site admin credentials, with no luck.