Well, Zorin has recently issued updates and I Believe more than one set of updates in the last month.
There may be a problem with your Update Notifier.
A few things you can try- Since we have checked your sources list and checked your update settings in Software & Updates:
In home folder, (hit ctrl+h) rename the ~/.config folder ".config-bkp"
This is just to create a back up of your configuration files in case you need them.
Then reboot. This process will set all configurations to default and a new .config folder will be created by the system on reboot. IF there were any changes to .config that were made that caused this issue, this would revert that. You can do the same with ~/.local/share
Reinstall update notifier:
sudo apt-get remove --purge update-notifier
sudo apt-get install -y update-notifier