Cannot create nodes with Drupal default and Bootstrap

Fresh install Drupal 8.1.3
Fresh install Bootstrap 8.x-3.0-beta3
Set Bootstrap as Admin theme, uninstall all other themes

Content cannot be created or edit nodes.

Watchdog message is always:

• Warning: strlen() expects parameter 1 to be string, array given in Drupal\Component\Utility\Unicode::validateUtf8() (line 686 of /var/www/d8/core/lib/Drupal/Component/Utility/Unicode.php).
• Warning: stristr() expects parameter 1 to be string, array given in Drupal\editor\EditorXssFilter\Standard::filterXssDataAttributes() (line 102 of /var/www/d8/core/modules/editor/src/EditorXssFilter/Standard.php).
• Notice: Array to string conversion in Drupal\Component\Utility\Html::load() (line 263 of /var/www/d8/core/lib/Drupal/Component/Utility/Html.php).

This seems to be a similar issue to https://www.drupal.org/node/2656572 but in the other issue there are entity references involved. But the errors above also occur, when no reference fields are present in a node form.