After receiving the update to 1.64.7704 beta, I waw the error "Choose an account / Unable to sync".
Resolved by uninstalling and reinstalling the beta version of To-Do.
On opening To-Do, I saw 'Choose an account', and then the account details for my main office365 account, along with an orange circle with message "unable to sync". Retried several times under different networks, wifi and mobile unsuccessfully.
Resolved this by uninstalling To-Do app, and reinstalling same beat version v1.64.7704. On opening the app I was prompted with a list of several different accounts to pick from, selected my main account and was able to get in successfully. All working again after a reinstall.