I have been using Apple Calendar on Mac for more than 10 years and have never had a problem with it syncing 5 calendars between iCloud and Google ones.
I installed Sequoia 15.1.1 and some days ago updated to 15.2. Today I tried to do something so simple as create an event on an iCloud calendar and the app just froze. Force quit / restart / nothing helps.
I have read the different solutions offered by other posters. Something is very clear to me:
macOS Sequoia and iOS 18 have been THE WORST upgrades ever to be produced by Apple. The number of bugs is unreliable, the so-called “improvements” to apps that were working perfectly fine, and that no-one was asking to change is just atrocious. Apps on iOS like the torch and Photos received updates that degraded them 10 years into the past.
The days of “It just works” are so long gone! I don't understand what the people working today at Apple are doing to the company. The user experience has gone downhill due to the terrible QC of the software on these last OS updates.