Apologies for the issues you're having with the My Account app. I know you've already tried to uninstall and re-install with no luck, but could you try again following the steps below:
Go into settings > apps > Xfinity MyAccount app (may need to tap "Storage" to get to 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.