I just worked in my notebook: https://cloud.sagemath.com/projects/e58ded0f-c359-46fe-86d1-f6c6184e1be4/files/Lab%203.ipynb. And after the latest checkpoint I can't do anything else - it begin autosaving and show me blank notebook. Or, sometimes, not blank, but very early version of this notebook. I revert to chekpoint, typing two dozens of signs - and this story repeat.