Hi Julien,
first let me thank you for your help!
Secondly – First I thought, that the problem is, that I am using custom post types, but it is the same with normal wordpress posts.
What did I do?
I installed awesome support and jetpack. I activated awesome support, and did set up everything. “Multiple Products”, “Plugin Pages” are the predefined ones.
I then activated wordpress jetpack and activated “Sharing” and “Publicize”. I then did connect everything with Facebook, Twitter and Google+. After this you’ll see a new option when you publish new posts, on the right hand side. There you can activate or deactivate sharing with social networks. After this I did write and publish a post, and the problem accured appeared.
Please let me know if you need anything else!
Those are my installed plugins:
Admin Menu Editor by Janis Elsts version 1.4.2,
Advanced Custom Fields by Elliot Condon version 4.4.0,
amr shortcode any widget by anmari version 2.4,
Awesome Support by ThemeAvenue version 3.1.4,
bbP private groups by Robin Wilson version 2.5.4,
bbPress Pencil Unread by G.Breant version 1.0.9,
bbPress by The bbPress Community version 2.5.4,
BuddyPress Security Check by Shea Bunge version 1.3.1,
Breadcrumb NavXT by John Havlik version 5.2.0,
rtMedia for WordPress, BuddyPress and bbPress by rtCamp version 3.7.33,
Buddypress Xprofile Custom Fields Type by donmik version 2.1.5,
BuddyPress by The BuddyPress Community version 2.2.1,
CodeStyling Localization by Heiko Rabe version 1.99.30,
Contact Form 7 by Takayuki Miyoshi version 4.1,
Custom Login by Austin Passy version 3.2,
TeamSpeak3 Webviewer for WordPress by Maximilian Narr version 1.3.3,
GD bbPress Attachments by Milan Petrovic version 2.1,
Google XML Sitemaps by Arne Brachhold version 4.0.8,
HumanCaptcha by Outerbridge by Outerbridge version 2.1,
Iframely by Itteco Corp. version 0.2.5,
Jetpack von WordPress.com by Automattic, Inc. version 3.3.1,
Max Mega Menu by Tom Hemsley version 1.5.3,
Members by Justin Tadlock version 0.2.4,
myCRED by Gabriel S Merovingi version 1.6.1,
Nav Menu Roles by Kathy Darling version 1.6.4,
Really Simple CAPTCHA by Takayuki Miyoshi version 1.8.0.1,
Regenerate Thumbnails by Viper007Bond version 2.2.4,
Relevanssi by Mikko Saari version 3.3.8,
Remove Dashboard Access by Drew Jaynes (DrewAPicture) version 1.1.2,
Require Featured Image by Press Up version 1.0.3,
Wordfence Security by Wordfence version 5.3.6,
WP Wartungsmodus by Designmodo version 2.0.3,
WP Social Sharing by Arjun Jain version 1.6,
WPFront Scroll Top by Syam Mohan version 1.4.2