You likely created all the users while Authentication is set to something other than "local". The readme states that you need to switch to local first.
Or, you didn't map the config folder correctly, so all your data is lost after an update.
We don't control the openvpn-as webui, it's upstream. You can make a request to them for the import/export functionality. But just so you know. When you update/recreate the container, it automatically exports your old settings and database and imports into the new container (as long as the config folder is mapped correctly).