evilblubb

@brettshumaker: Thanks for the Mail. I appreciate the time you spent on this problem.
For the other people with the problem:
For now the solution is to add add_theme_support( 'post-thumbnails' ) to your child-themes funtions.php.
A better solution should be around sometimes (again thanks brettshumaker for the time you are spending on this).

Sorry for the late answer, had a rough week.
Deleting the add_theme_support( 'post-thumbnails' ); from my child theme gives me “Warning: call_user_func() expects parameter 1 to be a valid callback, function ‘sslp_staff_member_warning_meta_box’ not found or invalid function name in ../wp-admin/includes/template.php on line 1037” with wp_debug = true.
Switching back to the tweentyfourteen theme gets me full functionality again.
I think the plugin is looking for the add_theme_support( 'post-thumbnails' ); only within the child themes functions.php.
If it helps I can send you the child theme I am using.

Okay this is a kind of weird, I am using a child theme for tweentyfourteen (didn’t tinker with the add_theme_support).
The add_theme_support( ‘post-thumbnails’ ); is already set in the tweentyfourteen theme functions.php, but adding this to the child themes functions.php resolves this.
I am wondering what was changed from 1.16 (where the stuff worked “out of the box”) and 1.19 (where I have to edit the functions.php in my child theme).

So I just checked 1.17 and 1.18.
V1.17 seems to break the whole plugin for me as I only get the option to set a Titse and an excerpt.
V1.18 gives me the same problem as V1.19.
When I upload a thumbnail with V1.16 it is displayed on the website. After Updating to V1.19 the thumbnail is still shown on the website, as before, but I now can’t delete or change the image anymore.

I am using the twentyfourteen theme. Ther was no error by itself, just an empty space where the upload prompt should be.
I tried using the plugin on a local and clean install with the same theme got me this:
“Warning: call_user_func() expects parameter 1 to be a valid callback, function ‘sslp_staff_member_warning_meta_box’ not found or invalid function name in …\wp-admin\includes\template.php on line 1037”.
Checking the functions.php showed me that post-thumbnails already is included in the theme.
With Version 1.16 it is working like I hoped it would. I didn’t had the time to check the versions in between. I will search if the files are still somewhere around.

No,
in german its called “Impressum”, thought imprint was the english synonym for that. Seems to be that “legal informations” is more common?
I ment a page with all the declarations you need in Germany if you are using google adsense, analytics etc.
Something like this -> https://www.uni-freiburg.de/legal-info.html