I have never seen this message, but I have experienced it more than once that the “emergency backup” created by Nuendo was corrupt and did not work.
I like Nuendo a lot. But since I started using it (again), I have become paranoid: I automatically save a project every 5 minutes. Additionally, I save manually after important steps. And that according to the principle “A/B”: I create another project file in the same folder at the beginning with “Save as…” and always alternate which one I choose when saving. This has often saved my day.
We also still use Sequoia here. It has never crashed. Nuendo but often already several times a day. Including broken back-up files.