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This behavior started after the ATT update was applied a week or so ago (early Aug 2018). Does not seem to matter which screen button path is used to launch the camera (side quick launch button is set to Clock). Occasionally the Camera is then 'active' upon the completion of reboot- but not always 100% usable - will often cause a reboot again once an attempt to open camera App screen (bottom square button selections).
In addition I am now seeing a SIM Card missing notification, but only since that Aug 2018 ATT update. Will check the phone to see is SIM Card was jarred loose in a drop since that update (I have case and phone has only been dropped a couple of feet- ever).
Both issues may be interconnected and eventually clear up, but issue(s) returns just as easily with no apparent rhyme or reason (typically phone is used indoors & on or off charger). I am also wondering if this behavior is a heat or moisture issue (very hot and humid now on Texas Gulf Coast). I do not recall adding any new Apps, since that ATT Aug 2018 update. But a lot of the Apps themselves seem to update every few weeks, so could be an App related issue causing reboot upon selection of Camera.