Log flooded by "This recurrence rule does not generate any valid instances"

Steps to reproduce

1. Subscribe to this public calendar which exhibits the issue
2. Wait for a bit and look at the log in the admin settings
3. I see several instances of this every 10 minutes or so, from different subscriptions (seems I have multiple users subscribing to this calendar or some other calendar with the same issue)

Expected behavior

Log is not flodded with messages about this, the calendar seems good to me when I read it e.g. with my mail client.

Actual behavior

Several dozen lines every attempted calendar sync land in the log with "Forbidden" in front

Calendar app version

5.0.10

CalDAV-clients used

DAVx5, Evolution

Browser

Firefox 135.0

Client operating system

Debian 12

Server operating system

Debian 12

Web server

Nginx

Database engine version

PostgreSQL

PHP engine version

PHP 8.0

Nextcloud version

30.0.6

Updated from an older installed version or fresh install

Updated from an older version

List of activated apps

Nextcloud configuration

Web server error log

Log file

Browser log

Additional info

Raw details of one entry:

{
    "reqId": "1oAqWLIzknF2uRJPIU6y",
    "level": 2,
    "time": "2025-02-15T16:41:31+00:00",
    "remoteAddr": "",
    "user": "--",
    "app": "dav",
    "method": "",
    "url": "--",
    "message": "Unable to denormalize calendar object from subscription 28",
    "userAgent": "--",
    "version": "30.0.6.2",
    "exception": {
        "Exception": "Sabre\\DAV\\Exception\\Forbidden",
        "Message": "This recurrence rule does not generate any valid instances",
        "Code": 0,
        "Trace": [
            {
                "file": "/var/www/montecristosoftware/nextcloud/apps/dav/lib/CalDAV/WebcalCaching/RefreshWebcalService.php",
                "line": 104,
                "function": "getDenormalizedData",
                "class": "OCA\\DAV\\CalDAV\\CalDavBackend",
                "type": "->"
            },
            {
                "file": "/var/www/montecristosoftware/nextcloud/apps/dav/lib/BackgroundJob/RefreshWebcalJob.php",
                "line": 73,
                "function": "refreshSubscription",
                "class": "OCA\\DAV\\CalDAV\\WebcalCaching\\RefreshWebcalService",
                "type": "->"
            },
            {
                "file": "/var/www/montecristosoftware/nextcloud/lib/public/BackgroundJob/Job.php",
                "line": 61,
                "function": "run",
                "class": "OCA\\DAV\\BackgroundJob\\RefreshWebcalJob",
                "type": "->"
            },
            {
                "file": "/var/www/montecristosoftware/nextcloud/apps/dav/lib/BackgroundJob/RefreshWebcalJob.php",
                "line": 66,
                "function": "start",
                "class": "OCP\\BackgroundJob\\Job",
                "type": "->"
            },
            {
                "file": "/var/www/montecristosoftware/nextcloud/lib/public/BackgroundJob/Job.php",
                "line": 46,
                "function": "start",
                "class": "OCA\\DAV\\BackgroundJob\\RefreshWebcalJob",
                "type": "->"
            },
            {
                "file": "/var/www/montecristosoftware/nextcloud/cron.php",
                "line": 162,
                "function": "execute",
                "class": "OCP\\BackgroundJob\\Job",
                "type": "->"
            }
        ],
        "File": "/var/www/montecristosoftware/nextcloud/apps/dav/lib/CalDAV/CalDavBackend.php",
        "Line": 3044,
        "message": "Unable to denormalize calendar object from subscription {subscriptionId}",
        "exception": [],
        "source": "https://calendar.google.com/calendar/ical/volteuropa.org_3qtptk1l0mfg76gq9nfqq1h4mg%40group.calendar.google.com/public/basic.ics",
        "CustomMessage": "Unable to denormalize calendar object from subscription 28"
    },
    "id": "67b0c49348908"
}

Start with apps/dav/lib/CalDAV/WebcalCaching/RefreshWebcalService.php at line 104 and apps/dav/lib/BackgroundJob/RefreshWebcalJob.php at line 73, following the refresh path to CalDavBackend.php line 3044. Reproduce the supplied subscription and inspect how the invalid recurrence exception is handled. Done means repeated syncs no longer flood the log while valid calendar data remains usable.