You will need to define some paths to where the data will live inside of the config for your ubooquity container. You can put the books wherever you want, but you need to make sure you setup the container with access to that place. In casaos, when setting up the Ubooquity container for the first time, it'll have a whole slew of little boxes for info you can put in that define what the container has access to. In there it should say "Storage" and this is where you put the file path to the books. I would recommend you put the books inside the Documents folder in your user directory, but you will also need to give the container access to a container-specific directory for storing configurations, themes, thumbnails, etc. I think that's in something like /DATA/ubooquity or something. CasaOS support should make this dir fairly obvious..

If you put the books in a directory called books inside your documents folder, the path you put into the container storage info would be "~/documents/books" where ~ represents your home directory. It's the same thing as typing /home/yourusername/documents/books

Have you set it up for each user? Each user has to be given specific access to each library by the administrator. Also, try just turning off users and having no login required?