Have you tried reinstalling Nuendo? (I suggest it because it helped “FCSCholz”.)
If this error suddenly appears out of the blue, it could be that an update of the operating system, for example, has changed or damaged a file that Nuendo needs. In this case, a reinstallation can repair the damage and Nuendo will work again. (As always, there is no guarantee that it will work for you. But it can’t hurt to try.)