My sagemathcloud worksheet reverts to a previous state

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Any operations with my notebook make it blank or return it to very early chekpoint. It just begin autosaving and then it happen. What can I do with it?

I just worked in my notebook: https://cloud.sagemath.com/projects/e.... And after the latest checkpoint I can't do anything else - it starts autosaving and show me blank notebook. Or, sometimes, not blank, but very early version of this notebook. I revert to chekpoint, type two dozens of signs - and this story repeats.