Thanks for reaching out and apologies the app is not working as it should! I know you've already tried to uninstall and re-install, but could you try again following the steps below:
Go into your device settings > apps > Xfinity MyAccount app (may need to tap "Storage" before the next step)
Clear app cache and data.
Once cleared, uninstall the app.
Reboot your device after you uninstall the app.
Once the device has booted back up reinstall the app.
Try to log in and let us know if you still have the same issue.