I opened a number of tabs at the same time. I think that's why Google Colab was not able to support the heavy load. The message stated:
Save failed
This file could not be saved. Please use the File menu to download the .ipynb and upload the notebook to make a copy that includes your recent changes.
Is downloading the file and uploading again the only solution?
It turned out to be a network issue. Output is heavy and colab is not able to save it is not able to make a handshake because internet speed is poor. So just switch the internet source and see it working.
I had the same problem despite ''Runtime > Change runtime type' being already set 'None' beforehand. As the OP I had no other instances of the notebook open.
In the end I did 3 things in the following order:
Just picking the 'Save' from the drop down again did the trick for me. My variables were still loaded all is working as before.
This can happen if you open the same notebook in multiple tabs and make incompatible edits to the notebook. At this point, the only way to save your work is to follow the advice in the dialog. To prevent this in the future, avoid simultaneously editing the same notebook in multiple browser windows.
I deleted (heavy) outputs and it worked out, it started saving without fail notifications
I got the same issue and eventually I found it's because I set up the hardware accelerator.
This would be fixed if reseting the 'Runtime/Change runtime type' to 'None'.
