1) Goto Settings
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2) Goto Applications > Manage Installed Applications
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3) Select Directv Now. Then select "Clear Data" and select "Clear Cache"
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4) Reboot the firestick by going to Settings > Device > Restart
BE CAREFUL and DO NOT select "Reset to Factory defaults"
5) Once rebooted relaunch your directv now app, sign in. And that worked for me.