So I tested the exact script in description in 15.10.8 and it doesn't reproduce anymore: I don't get any exception.
Now the result might not be the one expected: a document is moved to "NewSpace.NewPage" location, and the same document is also created back with a new title "NewTitle".
Now we do have also a warning log about using the deprecated method Document#rename(String). But it would have worked the same without the deprecation notice if we had used Document#rename(DocumentReference).

So I will close this as cannot reproduce since we don't get any error anymore: I consider that the probably expected result is more a misusage of the API.