[Bug]: Nextcloud Calendar allows/breaks the ical standard on recurring events (maybe?)

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

When marking a recurring task as completed in Vivaldi (Browser) Calendar (or via DAVx5), the resulting task is breaking several ical sync tools:

• DAVx5 ends up with a broken account if it was pushed by Vivaldi Calendar
• Python3 caldav lib also fails to sync the deleted item with:

  WARNING Ical data was modified to avoid compatibility issues
  (Your calendar server breaks the icalendar standard)
  This is probably harmless, particularly if not editing events or tasks
  (error count: 1 - this error is ratelimited)
  ---
  +++
  @@ -12,8 +12,7 @@
   SUMMARY:Rosie Wasser- und Filtercheck
   UID:4c83ec09-1fe3-47d4-9f59-70f439ae98b3
   STATUS:COMPLETED
  -COMPLETED;VALUE=DATE:20260521
  -END:VTODO
  +COMPLETED:20260521T120000ZEND:VTODO
   BEGIN:VTODO
   DTSTAMP;VALUE=DATE-TIME:20260521T192510Z
   DUE;VALUE=DA
  

• If DAVx5 tries to sync deleted completed tasks, it ends up with this error:
  "PUT /remote.php/dav/calendars/clouduser/personal/fa4d35ca-0902-434e-86a5-5d29f8a72af1.ics HTTP/1.1" 415 744 "-" "DAVx5/4.5.12-gplay (at.bitfire.davdroid)

Maybe the Vivaldi Calendar is sending a "not allowed" or conform date format? But Nextcloud should not allow non conform field values.
Marking a recurring item as completed work when it is done i.e. with DAVx5 but then deleting them fails.

Steps to reproduce

1. Connect Vivaldi Calendar to Nextcloud via iCal
2. Mark a recurring task as done
3. DAVx5 / Python3 caldav lib will now break on sync

Expected behavior

The Nextcloud server should send caldav items matching the definition.

Nextcloud Server version

33

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

{
    "system": {
        "defaultapp": "files",
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "33.0.3.2",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "forcessl": true,
        "theme": "",
        "maintenance": false,
        "maintenance_window_start": 2,
        "overwriteprotocol": "https",
        "default_phone_region": "ISO 3166-2:CH",
        "trusted_domains": [
            "cloud.oxi.ch"
        ],
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpmode": "smtp",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "ldapIgnoreNamingRules": false,
        "secret": "***REMOVED SENSITIVE VALUE***",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "memcache.local": "\\OC\\Memcache\\APCu",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "port": 6379
        },
        "loglevel": 2,
        "trashbin_retention_obligation": "90, 180",
        "htaccess.RewriteBase": "\/",
        "overwrite.cli.url": "https:\/\/cloud.oxi.ch",
        "auth.bruteforce.protection.enabled": false,
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpsecure": "ssl",
        "mail_smtpport": "465",
        "mail_smtpauth": 1,
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "updater.release.channel": "stable",
        "app_install_overwrite": [
            "keeweb",
            "forms"
        ],
        "encryption.key_storage_migrated": false,
        "apps_paths": [
            {
                "path": "\/var\/www\/html\/apps",
                "url": "\/apps",
                "writable": false
            },
            {
                "path": "\/var\/www\/html\/custom_apps",
                "url": "\/custom_apps",
                "writable": true
            }
        ],
        "mysql.utf8mb4": true,
        "memories.exiftool": "\/var\/www\/html\/custom_apps\/memories\/bin-ext\/exiftool-amd64-glibc",
        "memories.vod.path": "\/var\/www\/html\/custom_apps\/memories\/bin-ext\/go-vod-amd64",
        "enabledPreviewProviders": [
            "OC\\Preview\\Image",
            "OC\\Preview\\Movie"
        ],
        "enable_previews": true,
        "preview_max_memory": 256,
        "preview_concurrency_new": 4,
        "preview_ffmpeg_path": "\/var\/www\/html\/custom_apps\/recognize\/node_modules\/ffmpeg-static\/ffmpeg",
        "memories.vod.bind": "go-vod:47788",
        "memories.vod.connect": "go-vod:47788",
        "memories.vod.external": true,
        "memories.db.triggers.fcu": true,
        "memories.gis_type": 1,
        "upgrade.disable-web": true,
        "openmetrics_allowed_clients": [
            "10.5.20.0\/24"
        ]
    }
}

List of activated Apps

Enabled:
  - activity: 6.0.0
  - admin_audit: 1.23.0
  - app_api: 33.0.0
  - bruteforcesettings: 6.0.0
  - calendar: 6.4.1
  - circles: 33.0.0
  - cloud_federation_api: 1.17.0
  - comments: 1.23.0
  - contacts: 8.5.0
  - dashboard: 7.13.0
  - dav: 1.36.0
  - federatedfilesharing: 1.23.0
  - federation: 1.23.0
  - files: 2.5.0
  - files_3dmodelviewer: 0.0.16
  - files_downloadlimit: 5.1.0
  - files_pdfviewer: 6.0.0
  - files_reminders: 1.6.0
  - files_sharing: 1.25.2
  - files_trashbin: 1.23.0
  - files_versions: 1.26.0
  - firstrunwizard: 6.0.0
  - forms: 5.2.7
  - impersonate: 4.0.0
  - integration_google: 4.3.1
  - keeweb: 0.6.23
  - logreader: 6.0.0
  - lookup_server_connector: 1.21.0
  - memories: 8.0.1
  - nextcloud_announcements: 5.0.0
  - notes: 5.0.0
  - notifications: 6.0.0
  - notify_push: 1.3.3
  - oauth2: 1.21.0
  - password_policy: 5.0.0
  - photos: 6.0.0
  - polls: 9.1.2
  - previewgenerator: 5.13.0
  - privacy: 5.0.0
  - profile: 1.2.0
  - provisioning_api: 1.23.0
  - recognize: 11.0.1
  - recommendations: 6.0.0
  - related_resources: 4.0.0
  - serverinfo: 5.0.0
  - settings: 1.16.0
  - sharebymail: 1.23.0
  - support: 5.0.0
  - survey_client: 5.0.0
  - systemtags: 1.23.0
  - tasks: 0.17.1
  - text: 7.0.1
  - theming: 2.8.0
  - twofactor_backupcodes: 1.22.0
  - twofactor_totp: 15.0.0
  - updatenotification: 1.23.0
  - user_status: 1.13.0
  - viewer: 6.0.0
  - weather_status: 1.13.0
  - webhook_listeners: 1.5.0
  - workflowengine: 2.15.0
Disabled:
  - assistant: 3.4.2 (installed 3.4.2)
  - contactsinteraction: 1.14.1 (installed 1.11.0)
  - context_chat: 5.3.1 (installed 5.3.1)
  - encryption: 2.21.0 (installed 2.19.0)
  - files_external: 1.25.1 (installed 0.2.3)
  - suspicious_login: 11.0.0
  - testing: 1.23.0
  - twofactor_nextcloud_notification: 7.0.0
  - user_ldap: 1.24.0 (installed 0.6.1)

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

Additional info

This is the corresponding discussion for DAVx5

No response

No source file or test is named. Start by reproducing the recurring VTODO completion through Vivaldi Calendar or DAVx5, then inspect the DAV response and compare it with the iCal example and the Python caldav warning. Done means the server returns standards-compliant recurring items and DAVx5 can sync or delete completed tasks without a 415 error.