Hello there!
In that case, the next step would be to test if the backup self-contained everything it need to operate on itself, or not. If possible, install a clean, latest version of UE5 in a different PC or laptop, then deploy your project, and try to open if from there. If the issue persists with this setup, unfortunately it would mean that your project is missing critical data, and it may not be possible to re-open it.