There currently is a drupal bug: If you have your images in the public files directory without any subdirectory (what we all usually do have...) and if you have a convert command in your image style (e.g. convert to jpg - which we all usually do have) then you currently need a patch:
https://www.drupal.org/files/issues/2018-10-04/2630230.patch
apply in your drupal root with
patch -p1 < 2630230.patch
see https://www.drupal.org/project/drupal/issues/2630230