You can turn on Preferences / Launching Opus / Default Lister / Update Default Lister automatically when closing a Lister.
But the problem might be that it is already on, and you sometimes close a window when it isn't showing all your tabs, losing the tabs you want to see by default because the window you just closed saves over them. If so, turn that setting off off and use Settings > Set as Default Lister when you want to update your list of tabs, which will be kept forever until you explicitly tell Opus to change them again.
There should be no need to backup anything by hand as Opus doesn't randomly lose track of things. But if you really want to, the files are under /dopusdata\Layouts with the default lister stored in the System folder below there.
See How to backup or locate your Opus configuration (Simple) for more about config backups, locating the config folder, and the /dopusdata folder alias.