Some or all of your guests, and users no longer have access to vaults and folders they used to. What's changed?
This is usually caused by inadvertently using the third option in the Edit Access and Notification Settings dialog.
If you choose this option, all the settings displayed in this dialog above will be applied not only to this folder, but also to all subfolders.
For example, if you remove a user from this folder and then select this option, the user will be removed from not only this folder, but all subfolders as well
Once this setting is selected and applied, it cannot be reverted. The only way to restore the Users/Guest access to grant the permissions to the users again.