Unable to refresh my Calendar on MacBook Air and iPhone

On a MacBook Air M4 chip, not two months old, running Sequoia 15.5, and an iPhone 15 pro. All of a sudden, I'm having issues adding ics files to my calendars: it'll appear for a second, I add/accept, and then they disappear. I've troubleshot my internet connection, my logins for iCloud, and I cannot for the life of me understand why this is suddenly not working. But it's very inconvenient not being able to reliably add an event and have it appear on desktop/phone, including the fact that I'm not getting alerts about important events.

I'm getting the triangle exclamation icon, and "your calendar couldn't be refreshed," but any other event I add to it is fine. Not sure what to do.

[Re-Titled by Moderator]

Original Title: Calendar can't be refreshed; not a google calendar issue