This error seems to appear randomly to random people and is more frequent when people change their gamertag. The reason for the gamertag change is likely due to the gamertag listed under devices isn't properly updated upon changing your Xbox LIVE gamertag. Meaning sometimes when trying to log in again it will not recognize the gamertag listed with the new one. Any other reason is likely due to some internal error.
Solution to this is simple:
1.) Deactivate Xbox here: https://www.crunchyroll.com/acct/?action=devices
2.) Go back into app, sign in again.
This solution has worked with myself and a few others. It will reenter your Xbox under devices with the new gamertag (if changed) and it will also reset the device data and likely remove the data error that occurred when trying to sign in.
(Other references: I'm an Xbox LIVE ambassador and tried this solution myself.)
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