If you cannot get past sync / completed message when logging in, it is normally a problem with the local database on the device.
Sometimes a local database may get damaged and the App will need to be deleted and re-installed. This can be happen when a sync is interrupted before it completes due to a connection or device issue. In some case, logging in to the App as 2 different users on the same device can also cause the database to become damaged.
If you think you may have a damaged database you can try your login details on another device. This will tell you if there is a problem with your account or with the local data on your device.
Get a different device and make sure no one has logged into the App before.