Experienced this old chestnut follow 5.1.1 upgrade. Finally seemed to have fixed/corrected by performing cache reset using the follow:
Press and hold Power and Volume Down buttons simultaneously until you see something other than Google on the screen.
You should see a large arrow at the top of the screen.
Tap Volume Down repeatedly until you see Recovery in the arrow. Then tap the Power button
You should see an android on his back, chest open with a red triangle and exclamation mark.
While holding Power button tap Volume Up once then release Power.
You should now see a list of items at top of screen.
Tap Volume Down until the item to erase or wipe the cache partition is highlighted. Then tap Power button to initiate.
Status messages will appear at the bottom of the screen. Be patient – it can take 10-15 minutes to complete. When done restart the Nexus.
Press and hold Power and Volume Down buttons simultaneously until you see something other than Google on the screen.
You should see a large arrow at the top of the screen.
Tap Volume Down repeatedly until you see Recovery in the arrow. Then tap the Power button
You should see an android on his back, chest open with a red triangle and exclamation mark.
While holding Power button tap Volume Up once then release Power.
You should now see a list of items at top of screen.
Tap Volume Down until the item to erase or wipe the cache partition is highlighted. Then tap Power button to initiate.
Status messages will appear at the bottom of the screen. Be patient – it can take 10-15 minutes to complete. When done restart the Nexus.
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