Photo-Gallery Plugin keeps prompting for password

Hello,
yesterday I switched to the WordPress Photo Gallery Plugin from Web Dorade and it works well for logged-in users.

This is the link to the plugin:
https://web-dorado.com/products/wordpress-photo-gallery-plugin.html

When a user is not logged in (which is the standard case), then the user is prompted for the credentials upon clicking on a thumb nail preview image.

So all images are displayed correctly on the post as preview images, but as soon as a user clicks on an image in order to magnify it, he is presented with the “authentication required” prompt. I do not want this to happen.

I so use .htaccess to protect the admin area, but it should not affect the upload folder. Other images work. The media images are also stored in the /upload folder and they work fine. The problem occurs only with the images from the photo-plugin which are stored in /uploads/photo-gallery/ folder.

What could be the reason for this odd behavior?
Thank you very much in advance