File show placeholder image, even existing

The media/catalog/product/cache/18076855f65e9b9019a33b25468c9e1d/filename.png URLs are fetched by the pub/get.php

You can start debugging from there.

Interesting point is MagentoMediaStorageAppMedia::launch

See if there is an exception thrown

    } catch (Exception $e) {
        $this->setPlaceholderImage();
    }

This might give you more information. Unfortunately this exception is not logged by Magento.

Other debugging entry points

MagentoMediaStorageServiceImageResize::makeImage