Calendar events are auto-canceled for invited attendees

⚠️ 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 (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• Nextcloud Server is running on 64bit capable CPU, PHP and OS.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

When inviting someone to an appointment the event will be canceld by email notification after some time without any noteable reason. Its only canceld for attendees i invited but stays as scheduled in my calender. I don't know what could be the trigger.
I'm using the calender on my iPhone and also with CalDavSynchronizer and outlook.

Steps to reproduce

1. create an event and invite someone
2. invitation gets accepted
3. wait some time for the cancelation mail sent to the invited attendees

Expected behavior

no auto-cancelation

Installation method

Community Docker image

Operating system

Other

PHP engine version

PHP 8.0

Web server

No response

Database engine version

MariaDB

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

No response

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

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

Configuration report

{
    "htaccess.RewriteBase": "\/",
    "memcache.local": "\\OC\\Memcache\\APCu",
    "apps_paths": [
        {
            "path": "\/var\/www\/html\/apps",
            "url": "\/apps",
            "writable": false
        },
        {
            "path": "\/var\/www\/html\/custom_apps",
            "url": "\/custom_apps",
            "writable": true
        }
    ],
    "instanceid": "***REMOVED SENSITIVE VALUE***",
    "passwordsalt": "***REMOVED SENSITIVE VALUE***",
    "secret": "***REMOVED SENSITIVE VALUE***",
    "trusted_domains": [
        "my-domain",
        "my-host-ip",
        "my-hostname",
        "my-docker-ip"
    ],
    "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
    "overwrite.cli.url": "https:\/\/my-domain",
    "overwritehost": "my-domain",
    "overwriteprotocol": "https",
    "overwritewebroot": "\/nxtcld",
    "datadirectory": "***REMOVED SENSITIVE VALUE***",
    "dbtype": "mysql",
    "version": "24.0.6.1",
    "dbname": "***REMOVED SENSITIVE VALUE***",
    "dbhost": "***REMOVED SENSITIVE VALUE***",
    "dbport": "",
    "dbtableprefix": "",
    "mysql.utf8mb4": true,
    "dbuser": "***REMOVED SENSITIVE VALUE***",
    "dbpassword": "***REMOVED SENSITIVE VALUE***",
    "installed": true,
    "mail_smtpmode": "smtp",
    "mail_sendmailmode": "smtp",
    "mail_from_address": "***REMOVED SENSITIVE VALUE***",
    "mail_domain": "***REMOVED SENSITIVE VALUE***",
    "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
    "mail_smtpport": "465",
    "mail_smtpauth": 1,
    "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
    "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
    "mail_smtpauthtype": "LOGIN",
    "twofactor_enforced": "false",
    "twofactor_enforced_groups": [],
    "twofactor_enforced_excluded_groups": [],
    "maintenance": false,
    "log_type": "file",
    "logtimezone": "Europe\/Berlin",
    "logfile": "\/var\/log\/nextcloud.log",
    "loglevel": 0,
    "app_install_overwrite": [
        "calendar",
        "occweb",
        "keeweb",
        "files_reader",
        "documentserver_community",
        "sharerenamer"
    ],
    "theme": "",
    "mail_smtpsecure": "ssl",
    "updater.release.channel": "stable",
    "invitation_link_recipients": "yes",
    "default_phone_region": "DE",
    "data-fingerprint": "f31178f90b7c321b37aef44b60f47c29",
    "remember_login_cookie_lifetime": 1296000,
    "session_lifetime": 900,
    "session_keepalive": true,
    "auto_logout": true
}

List of activated Apps

Enabled:
 - accessibility: 1.10.0
 - activity: 2.16.0
 - apporder: 0.15.0
 - calendar: 3.5.1
 - cloud_federation_api: 1.7.0
 - comments: 1.14.0
 - contacts: 4.2.2
 - dav: 1.22.0
 - documentserver_community: 0.1.12
 - event_update_notification: 2.0.0
 - external: 4.0.0
 - federatedfilesharing: 1.14.0
 - federation: 1.14.0
 - files: 1.19.0
 - files_external: 1.16.1
 - files_pdfviewer: 2.5.0
 - files_rightclick: 1.3.0
 - files_sharing: 1.16.2
 - files_trashbin: 1.14.0
 - files_versions: 1.17.0
 - files_videoplayer: 1.13.0
 - keeweb: 0.6.9
 - logreader: 2.9.0
 - lookup_server_connector: 1.12.0
 - mail: 1.14.1
 - nextcloud_announcements: 1.13.0
 - notes: 4.5.1
 - notifications: 2.12.1
 - oauth2: 1.12.0
 - occweb: 0.1.0
 - onlyoffice: 7.5.6
 - photos: 1.6.0
 - privacy: 1.8.0
 - provisioning_api: 1.14.0
 - settings: 1.6.0
 - support: 1.7.0
 - tasks: 0.14.5
 - text: 3.5.1
 - theming: 1.15.0
 - twofactor_backupcodes: 1.13.0
 - twofactor_nextcloud_notification: 3.4.0
 - twofactor_totp: 6.4.1
 - updatenotification: 1.14.0
 - viewer: 1.8.0
 - workflowengine: 2.6.0
Disabled:
 - admin_audit
 - circles: 22.2.0
 - contactsinteraction: 1.5.0
 - dashboard: 6.0.0
 - encryption
 - firstrunwizard: 2.13.0
 - integration_google: 1.0.8
 - password_policy: 1.14.0
 - recommendations: 0.4.0
 - serverinfo: 1.14.0
 - sharebymail: 1.12.0
 - survey_client: 1.12.0
 - systemtags: 1.14.0
 - user_ldap
 - user_status: 1.0.1
 - weather_status: 1.2.0

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

download the log from admin panel but it has 80mb and site crashes when i try to copy it here

Additional info

docker is running on QNAP NAS

Start with the calendar app's invitation flow and reproduce the cancellation using an iPhone or CalDavSynchronizer with Outlook as described. Inspect the Nextcloud logs around the cancellation; the report says the log is available but does not provide it. Done means accepted invitations remain scheduled for attendees and no unexpected cancellation email is sent.