I just discovered, from flexibits help section, that if you use fantastical and zoom on >1 Macs it will force you to reauthenticate zoom every time you switch.
Using Zoom with Fantastical on multiple devices
Fantastical is only able to sign in to Zoon on one Mac and one iOS device. If you have two Macs and sign in to Zoom on both Macs you will need to reauthenticate with Zoom whenever you want to use it on a particular Mac. This is a limitation in how Zoom’s device authentication works.
I have 2 macs and switch between them often. And now I know why that little red exclamation mark keeps popping up.
Hope this helps.