For those who don't know, I remove older dropbox folders when I upload/post new folders. Just to be clear: I will remove the 403 folders and upload the 405 folders on the 5th of May. If you want to keep the content inside the folders, save/copy the link of the folder or the content from the folder to your own devices.
If you cannot find the folders: Go to my pixiv fanbox main page (the one with all the posts) and keep scrolling down. You will find posts with dropbox folder links. Depending on your tier, you will have access to them. Copy the links and save them or copy all the items in the folder and save them on your own device/computer/storage.