JulianLong has replied to your question Microsoft Sudoku Access Lost in Microsoft Community.
Just came across a fix that has worked
Go to your userprofile\AppData\Local\Packages\Microsoft.XboxIdentityProvider_cw5n1h2txyewy\AC\TokenBroker
If there are Accounts and Cache folders available delete them. (I renamed them to be on the safe side) But this worked well.
Now try starting Xbox and signing into Sudoku.