I had same problem and I found out I was logged out from Jupyter. I found that when I went to Jupyter home page and it asked me to enter password. After I entered password I could save my notebook (it was still running in other tab).
I had same problem and I found out I was logged out from Jupyter. I found that when I went to Jupyter home page and it asked me to enter password. After I entered password I could save my notebook (it was still running in other tab).