CalDAV stopped working with Apple Calendar after recent macOS update (works in Thunderbird)

Hello,

I’m running macOS 26.3.1 (25D2128) and noticed that CalDAV calendars no longer work correctly in the Apple Calendar app, even though the same server works perfectly with other clients.

Details:

• Calendar server: Nextcloud CalDAV
• macOS version: 26.3.1
• Client with issues: Apple Calendar
• Client working: Thunderbird

Behavior:

• The CalDAV calendar previously worked fine with Apple Calendar.
• After recent macOS updates the calendar either fails to connect or does not sync properly.
• The exact same CalDAV URL and credentials work immediately in Thunderbird.

Connection details:

• Server URL: https://<server>/remote.php/dav/principals/users/<username>/

or

• Server URL: https://<server>/remote.php/dav/calendars/<username>/<calendar>/

Configuration method used in macOS:

System Settings → Internet Accounts → Add Account → Other → CalDAV (Manual)

Username and password are correct and verified.

Troubleshooting already done:

• Removed and re-added the CalDAV account
• Verified TLS certificate
• Confirmed server works with Thunderbird
• Verified Nextcloud calendar works via web interface

Because the same server works with other CalDAV clients, this appears to be a Calendar.app regression in recent macOS updates.

Has anyone else experienced this with CalDAV / Nextcloud / self-hosted calendars on recent macOS versions?

Any known workaround?

Thanks!