Can you please open a terminal and run
sudo apt install --reinstall gnome-calendar
sudo apt install gnome-tweaks
Log out and in, and test gnome calendar. It it is still not working, open Tweaks from the app menu and go to the Extensions tab.
Disable all gnome-extensions. Then test Calendar. If it works, can re-enable extensions, testing calendar as you go, until you find which one is interfering with the app.
If problem persists after reinstalling and disabling all extensions... I'm not sure yet. 
then quickly closed it 