URL is not sharable... whats the issue here?

I have a very strange issue:
on the site http://finanz-erfolgscoaching.de there are several pages and posts. The posts (and ONLY the posts) are NOT shareable on most Social Media sites.
(example Google+ see screenshot. FYI any other URLs from that domain are sharable, but NOT the blog posts).
An example post so you can see the issue is this one: http://finanz-erfolgscoaching.de/anlagestrategie-bei-negativzinsen-und-inflation/
Anyone has an idea what could be the problem?