People are not authorized to share my posts on facebook. help!

I have been having a problem the past few days with a blog I built for a friend of mine. The blog is about a year old and always functioned the way it should. But a couple of days ago Facebook started giving some problems.

Below each blog post there is a Facebook 'like' button. Everyone can click this button and the post will be shared on their profile. No problems there.
But then, when someone clicks the 'share' button below the post on Facebook, they get a message saying: 'You do not have sufficient permissions to do that'.

I have asked several people to test this and everyone had the same problem and only with my friends site.

But this only happens if the post is shared on Facebook through the like button below the post on the blog. If someone manually posts the post-url on Facebook, then nobody has a problem further sharing this on their profiles.

I have tried installing a different plugin, but still got the same problem.

If you want, you can see for yourself here (it's a dutch blog). Click the like button below the post and then go to Facebook and try to share the link you just liked.

Any help is very much appreciated as I don't know how to fix this.