iCloud calendars missing from sidebar

This is a new one for me. Something happened, and now although I can see "iCloud" listed on the left, in the sidebar, there are no calendars underneath. The calendars exist in iCloud. I can see them in the browser, and they are on my phone. For some reason they just don't appear on the Mac.

I tried wiping the ~/Library/Calendars folder, as well as deleting my iCal.plist. If I uncheck "Calendars" from the iCloud preference pane, or from Internet Accounts, the "iCloud" section goes away. If I re-check, it comes back, but without any calendars. Google calendars display just fine, it's only iCloud that's a problem.

Weirdly, when I go into Preferences in Calendar, and select Alerts, the Accounts dropdown shows "Google, iCloud, Group, Outlook, On My Mac". "Group" looks really suspicious.

I've rebooted several times. I signed out of iCloud entirely, and signed back in. Nothing seems to work.

In the console, for CalendarAgent I see some stuff repeated over and over that might be relevant, but it might also be a red herring:

Attempted to decode a collection type 'NSString' (subclass of 'NSObject') for key 'NS.keys'. 'NSString' requires its subclasses to be explicitly added to the allowed classes list but it is not present. Allowing this has been a source of security issues. Please ensure you meant this type to be in archives: 'NSString' (0x1d97105a8) [/System/Library/Frameworks/Foundation.framework]

*** -[NSKeyedUnarchiver validateAllowedClass:forKey:] allowed unarchiving safe plist type ''NSString' (0x1d97105a8) [/System/Library/Frameworks/Foundation.framework]' for key 'NS.keys', even though it was not explicitly included in the client allowed classes set: '{(

"'ICSProperty' (0x1d9a5f338) [/System/Library/PrivateFrameworks/iCalendar.framework]",

"'NSDictionary' (0x1d96e2560) [/System/Library/Frameworks/CoreFoundation.framework]",

"'NSArray' (0x1d96e2420) [/System/Library/Frameworks/CoreFoundation.framework]"

)}'. This will be disallowed in the future.