Why won't Doro Liberto 820 Mini boot past the Android screen?

It was working fine yesterday and I switched it off overnight as usual. When I switched it on this morning it refused to boot up. It sticks on the Android screen for about 20 seconds, clears the screen for a couple of seconds then the Android screen for another 20 seconds and so on. Removing and reconnecting the battery makes no difference. The usual Android trick of holding "Volume Up + Home + Power" simultaneously to go into safe mode has no effect. Anyone any other ideas?

My phone has exactly the same symptoms.  I have not been able to work out a solution.  Any further insight appreciated. 

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