Incomplete settings display. Shortcodes crashes site.

At first the settings page is displayed incomplete. It seems there is an exception which stops rendering the full page. The last output is:

<h3>Sonstige</h3>
<li class="block">
<label>
<input type="checkbox" value="1" name="dem[force_cachegear]"  />

There is also no save button.

Creating a new poll works but including the shortcode into a post crashes the page so it stops rendering before echoing the post body text.

The theme settings page is just a white box with nothing in it.

Wordpress 4.7.5 with german language. Maybe a language issue?