[Bug]: Warning 'headers already sent by ZipStreamer' when download folder in zip

[lucasguerrero-rd]

⚠️ 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 we download a folder/file in zip format, we receive this warning even though the download is proceeding successfully.

Steps to reproduce

1. Files
2. Select some folder
3. Three dots
4. Download

Expected behavior

Downloading a file should not trigger a warning.

Nextcloud Server version

32

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.3

Web server

Apache (supported)

Database engine version

PostgreSQL

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

None

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

{
    "system": {
        "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
            }
        ],
        "maintenance_window_start": 22,
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "overwriteprotocol": "https",
        "skeletondirectory": "",
        "objectstore": {
            "class": "\\OC\\Files\\ObjectStore\\S3",
            "arguments": {
                "bucket": "nextcloud-storage",
                "region": "eu-west-1",
                "hostname": "***REMOVED SENSITIVE VALUE***",
                "port": "9000",
                "storageClass": "STANDARD",
                "objectPrefix": "urn:oid:",
                "autocreate": false,
                "use_ssl": true,
                "use_path_style": true,
                "legacy_auth": false,
                "key": "***REMOVED SENSITIVE VALUE***",
                "secret": "***REMOVED SENSITIVE VALUE***"
            }
        },
        "mail_smtpmode": "smtp",
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "587",
        "mail_smtpauthtype": "LOGIN",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "upgrade.disable-web": true,
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "localhost",
            "***REMOVED SENSITIVE VALUE***"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "pgsql",
        "version": "32.0.0.13",
        "overwrite.cli.url": "https://localhost",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "memcache.distributed": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "password": "***REMOVED SENSITIVE VALUE***",
            "port": 6379
        },
        "forbidden_filename_basenames": [
            "con",
            "prn",
            "aux",
            "nul",
            "com0",
            "com1",
            "com2",
            "com3",
            "com4",
            "com5",
            "com6",
            "com7",
            "com8",
            "com9",
            "com\u00b9",
            "com\u00b2",
            "com\u00b3",
            "lpt0",
            "lpt1",
            "lpt2",
            "lpt3",
            "lpt4",
            "lpt5",
            "lpt6",
            "lpt7",
            "lpt8",
            "lpt9",
            "lpt\u00b9",
            "lpt\u00b2",
            "lpt\u00b3"
        ],
        "forbidden_filename_characters": [
            "<",
            ">",
            ":",
            "\"",
            "|",
            "?",
            "*",
            "\\",
            "/"
        ],
        "forbidden_filename_extensions": [
            " ",
            ".",
            ".filepart",
            ".part"
        ],
        "mail_sendmailmode": "smtp",
        "default_phone_region": "FR",
        "loglevel": 2,
        "maintenance": false,
        "mail_smtpsecure": "tls",
        "mail_smtpauth": false,
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "app_install_overwrite": [],
        "defaultapp": "files",
        "enable_previews": false
    }
}

List of activated Apps

Enabled:
- activity: 5.0.0-dev.0
- app_api: 32.0.0
- bruteforcesettings: 5.0.0-dev.0
- circles: 32.0.0
- cloud_federation_api: 1.16.0
- comments: 1.22.0
- contactsinteraction: 1.13.1
- dashboard: 7.12.0
- dav: 1.34.2
- federatedfilesharing: 1.22.0
- federation: 1.22.0
- files: 2.4.0
- files_downloadlimit: 5.0.0-dev.0
- files_pdfviewer: 5.0.0-dev.0
- files_reminders: 1.5.0
- files_sharing: 1.24.0
- files_trashbin: 1.22.0
- files_versions: 1.25.0
- firstrunwizard: 5.0.0-dev.0
- impersonate: 3.0.0
- logreader: 5.0.0-dev.0
- lookup_server_connector: 1.20.0
- nextcloud_announcements: 4.0.0-dev.0
- notifications: 5.0.0-dev.0
- oauth2: 1.20.0
- password_policy: 4.0.0-dev.0
- privacy: 4.0.0-dev.0
- profile: 1.1.0
- provisioning_api: 1.22.0
- recommendations: 5.0.0-dev.0
- related_resources: 3.0.0-dev.0
- richdocuments: 9.0.0
- serverinfo: 4.0.0-dev.0
- settings: 1.15.1
- sharebymail: 1.22.0
- sharelisting: 1.3.0
- support: 4.0.0-dev.0
- survey_client: 4.0.0-dev.0
- systemtags: 1.22.0
- text: 6.0.0-dev.0
- theming: 2.7.0
- twofactor_backupcodes: 1.21.0
- updatenotification: 1.22.0
- user_saml: 7.0.0
- user_status: 1.12.0
- viewer: 5.0.0-dev.0
- weather_status: 1.12.0
- webhook_listeners: 1.3.0
- workflowengine: 2.14.0

Disabled:
- admin_audit: 1.22.0
- encryption: 2.20.0
- files_external: 1.24.0
- photos: 5.0.0-dev.1 (installed 5.0.0-dev.1)
- suspicious_login: 10.0.0-dev.0
- twofactor_nextcloud_notification: 6.0.0-dev.0
- twofactor_totp: 14.0.0
- user_ldap: 1.23.0

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

{"reqId":"oJol8RODNneQ1P59rhvI","level":2,"time":"2025-10-17T14:26:54+00:00","remoteAddr":"10.214.33.1","user":"toto","app":"PHP","method":"HEAD","url":"/remote.php/dav/files/toto/Test/?accept=zip","message":"Cannot modify header information - headers already sent by (output started at /var/www/html/3rdparty/deepdiver/zipstreamer/src/ZipStreamer.php:325) at /var/www/html/3rdparty/sabre/http/lib/Sapi.php#64","userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/141.0.0.0 Safari/537.36","version":"32.0.0.13","clientReqId":"bb0495684a2230efa978e9b88b8a4c37","data":{"app":"PHP"}}

Additional info

I am creating this issue knowing that a thread on the community forum that was closed regarding this issue existed, but it was closed automatically without a concrete answer.

[szaimen]

Cc @susnux

[BJKle]

I have this too since I upgraded to the latest version of NC.
Regardless of downloading multiple files via Mac app, Windows app or directly via the web browser.

[agru]

We have also hundreds of warnings in the log. We also have Sapi.php#64 at the end of the warning, but also Sapi.php#68.

[camouflage81]

Same issue with Version
[Nextcloud Hub 25 Autumn] (32.0.6).
Hundreds of log entries.

[BJKle]

No change with current Version [Nextcloud Hub 26 Winter] (33.0.0).
Log still filling