The easiest way I found is this:
https://github.com/nteract/hydrogen/issues/922#issuecomment-405456346
Just open another (non-running, existing) notebook on the same kernel, and the issue is magically gone; you can again save the notebooks that were previously showing the _xsrf error.
If you have already closed the Jupyter home page, you can find a link to it on the terminal from which Jupyter was started.