For the record, my students got this error a couple times, on a relatively recent Ubuntu, with Sage 6.3, probably recompiled on site. Usually, this is due to Sage dying brutally (typically when they close the terminal where they launched the notebook). Removing the corrupted empty "users.pickle" file does the job to recover. But this is annoying, and beyond their technical skill if this happens when I am not arount.
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