How to fix 404 error for a specific folder while using Folder Gallery plugin

I got a WordPress site (latest version) in which I have installed a plugin named Folder Gallery. The plugin works fine, but I have started having weird 404 issues lately. The plugin's purpose is simple: create a lightbox gallery from a folder. The webmaster only have to drop the pictures on the server using FTP, and a simple shortcode, like so:

[foldergallery folder="local_path_to_folder" title="Gallery title"]

So in my case:

[foldergallery folder="wp-content/photos/2015/Quebec" title="Quebec City event photos"]

In my "2015" page in WordPress, I used that shortcode several times with great success (and it still works at the moment). Recently, I have added another shortcode to that same page. Result: All previous shortcodes still work, but the new one throws a 404 error.

I have then tried to call an image directly from that folder: 404 again. The path is correct. After googling for solutions, I have found several times the permalink fix in order to re-build the permalink structure. Did not work for me.

Some additional information:

• CHMOD is 777 for the folder and files (I intentionally made it 777 for testing purposes).
• I tried disabling all plugins, including the one I thought that would be the culprit, All In One WP Security & Firewall.
• I tried completely removing the .htaccess file from the site.
• The is no space in folders name and files name.
• I tried creating the folder from scratch and re-uploading all pictures from my computer (for what it's worth, I use FlashFXP).
• It works for other shortcodes in that same folder, and also for pictures from other folders as well.

I am now clueless and looking for thoughts.

Thank you for help.

via Chebli Mohamed