Just had this happen yesterday with N7 on Windows. Was attempting to replace audio in a video file. About halfway through the job, I got the alert.
Shut down Nuendo, reopened, tried it again, same problem. Restarted computer, same problem.
It was my first try to replace audio. Not sure what happened, but I hoped it would be a smoother process. The video editor is going to have to replace the audio I send.