I've just started using your plugin and I'm really impressed with it. I have one request though - can you add a rel='nofollow' attribute to the 'subscribe without commenting' links.
I've just noticed on my webmaster tools account that Google has been following the subscribe links to the 'manage subscriptions' pages, and suddenly every post on my site is generating a crawl error.
Your plugin isn't actually responsible for the crawl error - it's WordPress SEO, which I know you've had problems with before. WordPress SEO is misinterpreting your permalink structure and generating a false rel="canonical" link for the page which Google is then trying to follow.
That said, I don't think it's a good idea to let the Googlebot onto back-end pages like this at all, as there will always be the potential for these kinds of problems. A simple rel='nofollow' in the link would be a great start, but an even better solution if it's possible would be to add a 'noindex, nofollow' robots meta tag to all of the plugin's management pages as well.
Again, great plugin, so I hope this issue can be sorted out soon.