If you are unable to authenticate using Office 365, please try the following workaround changing the Authentication Mode from O365 to Web Log in.
At the Sites section, tap and hold on your SharePoint-configured portal name, then tap again on Edit Settings
Once the Settings window is displayed, what needs to be changed is the Authentication Mode:
The change needed is from Office 365 to Web Login
Once Web Login is the new Mode, be sure to select once again the Account to be used.