Well, if it's not the ebooks or the batteries, and assuming identical hardware, then it sure sounds like firmware or OS.
Not knowing if the firmware is a complete software upgrade, I wonder if the update may not have included a kernel or other software component that didn't get updated for everyone? Or if not everyone had all the same components depending on which firmware they updated from, etc. If so, some could have some leftover software components that worked OK with a former firmware, but not with the current. Who knows?
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