Hello sfraser1,
Thank you for using Apple Support Communities. It's our understanding that the Calendar app isn't working as expected after recently updating your Mac mini. We're happy to look into this with you.
Let's test the issue while in safe mode. Starting in safe mode allows your Mac to run certain checks, remove caches, and prevent login items from automatically launching. This can isolate possible third party issues. Use the steps and information in the link below for assistance with safe mode:
How to use safe mode on your Mac
We'd also like you to test the issue in another user account. This will help us determine if the issue is specific to your user account or system wide. Set up users, guests, and groups on Mac
Let us know the results. Be sure to include specific error messages and any other troubleshooting steps that you may have tried.
Have a great day!