Close the app (drag from the top of screen and throw the app off of the bottom of the screen) and re-launch it. Verify that your WiFi is working and that the app says that you're signed in. Check that you can access your account from Settings->Account in the app. Try toggling avatars off and then on again under Settings->Preferences. Reboot the tablet. Try uninstalling and re-installing the app one more time. Try refreshing Windows (under Settings->Change PC Settings->Update and recovery->Recovery). Give it a day or so to see if it resolves itself.
If all else fails, consider re-installing Windows, either by choosing Settings->Change PC Settings->Update and recovery->Recovery->"Remove everything and re-install Windows" or running Setup.exe again and, this time, choosing "Keep Nothing." A couple of other people have reported similar minor issues after upgrading that clean installations resolved.