Preview does not work when sharing a single image

For whole folders being shared previews are generated well (using the gallery app). However, for single shared images (photos) only a small cross appears as can be seen in attached screenshot

I tried several options in the config.php file already.
These are my current sharing settings in config.php:

  'memcache.local' => '\\OC\\Memcache\\ArrayCache',
  'tempdirectory' => '/home/joephein/tmp/nextcloudtemp',
  'enable_previews' => true,
  'preview_max_scale_factor' => 1,
  'preview_max_x' => 1024,
  'preview_max_y' => 1024,

Nextcloud 11.0.3 (stable)
Users: 6
Files: 74488
PHP Version: 5.6.28
Memory Limit: 512.0 MB
Max Execution Time: 3600
Upload max size: 511.0 MB
Type: mysql
Version: 10.1.23
Size: 88.6 MB

The output of your Nextcloud log in Admin > Logging: only gives two error messages that seem to be related to the webcron job:

Error PHP Undefined index: REQUEST_URI at /home/joephein/public_html/lib/base.php#177 2017-05-15T14:00:05+0200
Error PHP Undefined index: REQUEST_METHOD at /home/joephein/public_html/lib/private/Route/Router.php#79