Server responded with 403

How do you exactly use these modules?
What is your TinyPNG module (/admin/config/tinypng) configuration? Is the "Compress on upload" enabled? Is "Integration Method" value is "Download" or "Upload"?
Is your File field path is configured to use the private:// path?
How your image field exactly configured?
When does this error usually appears?

Thank you for a quick reply. You're awesome!

How do you exactly use these modules?
I have several content types with image fields (I'm not using the Media library on these fields, to be extra clear). On every field TinyPNG used to shrink the uploaded images.

What is your TinyPNG module (/admin/config/tinypng) configuration? Is the "Compress on upload" enabled? Is "Integration Method" value is "Download" or "Upload"?
Download.

Is your File field path is configured to use the private:// path?
Yes. The settings at admin/config/media/file-system/filefield-paths is set to private://filefield_paths and in settings.php I use a path to a folder outside the www, chmodded to 777.

How your image field exactly configured?
It's set up the most standard way possible. The only addition is the File Field Paths which I use to change the filename based on information in other fields on the content type.

When does this error usually appears?
On every image upload. The watchdog log says "access denied", and then renders the message Path: /system/files/filefield_paths/image.png. Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException: in Drupal\system\FileDownloadController->download() (line 92 of /home/kulhisto/domains/reklamfranforr/www/web/core/modules/system/src/FileDownloadController.php). which makes me think that the awesome TinyPNG-module and service can't access the image due to access restrictions.
The image gets uploaded allright, and a thumbnail is shown, but the image doesn't get shrunk via TinyPNG.