It's interesting that it isn't working for everyone. It is now working for me on 13.4.1. Then again, the solution of deleting files from ~/Library also worked for a few days until it stopped, so I'll save the champagne until Monday.
The change log for 13.4.1 does not mention bug fixes, just "This update provides important security fixes and is recommended for all users."
I did create a ticket and continued escalating this through to senior tech support who looked through my logs and didn't find anything interesting. She eventually realized that this is a bug and a defect had been open in their system for this.
However, something else is going on. If I go to
Calendar->Accounts->Add Accounts...->Add Other Account->CalDav acccount->Change Account Type to 'advanced'->Cancel
The dialog to create an account remains on the screen, but is empty. There's no way to close or cancel from it except for Force Quit. Oddly, this does not happen if I begin the same process from Calendar->Add Account...