I have been having trouble with the Microsoft To-Do app for Android for a while now. After signing in as usual, the app prompts me to "Try signing in again" and provides an option to try again. After trying again, my account appears in the list however there is an orange indicator below it that states "Unable to Sync".
If I attempt to sign into that account, the app hangs on the "Signing you in..." and no further progress is made. Occasionally the app will appear to sign me in and progress to the main To-Do screen, however I am kicked back to the account list screen within a few seconds.