I have activated the secure mode of your plugin and everything works well, part from my default user avatar not loading anymore.
I am using the plugin Default Gravatar Sans to block all calls to Gravatar
And then Simple Local Avatars to provide a new default avatar and an upload function for the registered users.
I am not sure where to start now, what could cause the issue?
Edit: I´ve already deleted the hotlinking part from your htaccess file (because my images are being served from several subdomains...) but that did not help either.