[Bug]: Web-based updater does not provide relevant error info after failure

⚠️ 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

Nextcloud version: 28.0.1
Operating system: Ubuntu 22.04
Web server: Nginx 1.24
PHP: 8.1 fpm

Web GUI updater fails to successfully complete, but displays error message. However, no information is provided by error message because it only contains raw HTML code (see attached screenshot).

Steps to reproduce

1. See notification of availability of update in Nextcloud server Web-based GUI
2. Go to administrative settings panel
3. Execute update
4. Update fails (due to unknown reason)
5. Error message provides no helpful information - see screenshot below:

Expected behavior

Error message should provide helpful information, not raw HTML code.

Installation method

None

Nextcloud Server version

26

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": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "collaboration.cafe"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "28.0.1.1",
        "overwrite.cli.url": "https:\/\/collaboration.cafe",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "default_phone_region": "40",
        "default_language": "en",
        "force_language": "en",
        "force_locale": "en_US",
        "allow_user_to_change_display_name": true,
        "check_for_working_wellknown_setup": false,
        "enable_previews": true,
        "enabledPreviewProviders": [
            "OC\\Preview\\PNG",
            "OC\\Preview\\JPEG",
            "OC\\Preview\\GIF",
            "OC\\Preview\\BMP",
            "OC\\Preview\\XBitmap",
            "OC\\Preview\\OpenDocument",
            "OC\\Preview\\Krita",
            "OC\\Preview\\Movie",
            "OC\\Preview\\PDF",
            "OC\\Preview\\MP3",
            "OC\\Preview\\TXT",
            "OC\\Preview\\SVG",
            "OC\\Preview\\MarkDown",
            "OC\\Preview\\Illustrator",
            "OC\\Preview\\HEIC",
            "OC\\Preview\\MSOffice2003",
            "OC\\Preview\\MSOffice2007",
            "OC\\Preview\\MSOfficeDoc",
            "OC\\Preview\\Photoshop",
            "OC\\Preview\\Postscript",
            "OC\\Preview\\StarOffice",
            "OC\\Preview\\TIFF",
            "OC\\Preview\\Font"
        ],
        "memcache.distributed": "\\OC\\Memcache\\Redis",
        "memcache.local": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "port": 6379
        },
        "knowledgebaseenabled": false,
        "cron_log": true,
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpmode": "smtp",
        "mail_sendmailmode": "smtp",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "587",
        "defaultapp": "files",
        "mail_smtpauth": 1,
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "maintenance": false,
        "app_install_overwrite": [
            "files_ebookreader",
            "files_reader",
            "epubreader",
            "music"
        ],
        "updater.secret": "***REMOVED SENSITIVE VALUE***"
    }
}

List of activated Apps

Enabled:
  - activity: 2.20.0
  - calendar: 4.6.4
  - circles: 28.0.0-dev
  - cloud_federation_api: 1.11.0
  - comments: 1.18.0
  - contacts: 5.5.1
  - contactsinteraction: 1.9.0
  - dashboard: 7.8.0
  - dav: 1.29.1
  - federatedfilesharing: 1.18.0
  - federation: 1.18.0
  - files: 2.0.0
  - files_pdfviewer: 2.9.0
  - files_reminders: 1.1.0
  - files_sharing: 1.20.0
  - files_trashbin: 1.18.0
  - files_versions: 1.21.0
  - firstrunwizard: 2.17.0
  - integration_mastodon: 2.0.3
  - logreader: 2.13.0
  - lookup_server_connector: 1.16.0
  - mail: 3.5.5
  - music: 1.10.0
  - nextcloud_announcements: 1.17.0
  - notifications: 2.16.0
  - oauth2: 1.16.3
  - password_policy: 1.18.0
  - photos: 2.4.0
  - previewgenerator: 5.4.0
  - privacy: 1.12.0
  - provisioning_api: 1.18.0
  - recommendations: 2.0.0
  - related_resources: 1.3.0
  - richdocuments: 8.3.1
  - serverinfo: 1.18.0
  - settings: 1.10.1
  - sharebymail: 1.18.0
  - spreed: 18.0.3
  - support: 1.11.0
  - survey_client: 1.16.0
  - systemtags: 1.18.0
  - text: 3.9.1
  - theming: 2.3.0
  - twofactor_backupcodes: 1.17.0
  - updatenotification: 1.18.0
  - user_status: 1.8.1
  - viewer: 2.2.0
  - weather_status: 1.8.0
  - workflowengine: 2.10.0
Disabled:
  - admin_audit: 1.18.0
  - audioplayer: 3.4.1 (installed 3.4.1)
  - bruteforcesettings: 2.8.0
  - encryption: 2.16.0
  - epubreader: 1.4.7 (installed 1.4.7)
  - files_external: 1.20.0
  - richdocumentscode_arm64: 23.5.705 (installed 23.5.705)
  - suspicious_login: 6.0.0
  - twofactor_totp: 10.0.0-beta.2
  - user_ldap: 1.19.0

Nextcloud Signing status

https://collaboration.cafe/index.php/settings/integrity/failed

No errors have been found.

Nextcloud Logs

No response

Additional info

None to provide.

Reproduce the failure through the Nextcloud web-based updater using the listed administrative settings steps, then trace the updater's error-display path. The fix is complete when an update failure shows helpful error information instead of raw HTML; no relevant source file or test is named, and the report provides no logs.