You will face a problem to unshare a folder if your shared photo folder is too large in Dropbox. You may take more time for the unsharing operation over a normal web request. If this happens, move your photos from shared folder to another (not shared) folder. Dropbox will take time to finish syncing process. Unshare the (now empty) shared folder. Move your photos back into the unshared folder or delete the unshared folder and use the new folder. Now you can easily share the photos.