[Plugin: Efficient Related Posts] Won't activate

When I try and activate the link from the plugins page, my browser just spins and eventually spits me out to this URL:

http://www.example.com/wp-admin/plugins.php?action=activate&plugin=efficient-related-posts/efficient-related-posts.php&plugin_status=all&paged=1&_wpnonce=d8bbd0b2d6

Which is a page that doesn’t exist so I get a 404.

I was using YARPP, but I’ve deactivated and uninstalled that plugin. Any ideas what might be happening?