[Bug]: `hash_hkdf(): Argument #2 ($key) cannot be empty` / `HMAC does not match`

⚠️ 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

After upgrading my server from Nextcloud v24.0.4 to v24.0.5 and also upgrading to PHP8 (I don't know if this is of importance), the Nextcloud cron job generates the following error output:

ValueError: hash_hkdf(): Argument #2 ($key) cannot be empty in .../lib/private/Security/Crypto.php:97
Stack trace:
#0 .../nextcloud/lib/private/Security/Crypto.php(97): hash_hkdf('sha512', '')
#1 .../nextcloud/lib/private/Security/IdentityProof/Manager.php(110): OC\Security\Crypto->encrypt('-----BEGIN PRIV...')
#2 .../nextcloud/lib/private/Security/IdentityProof/Manager.php(133): OC\Security\IdentityProof\Manager->generateKey('user-juergen')
#3 .../nextcloud/lib/private/Security/IdentityProof/Manager.php(146): OC\Security\IdentityProof\Manager->retrieveKey('user-juergen')
#4 .../nextcloud/lib/private/Security/IdentityProof/Signer.php(64): OC\Security\IdentityProof\Manager->getKey(Object(OC\User\User))
#5 .../nextcloud/apps/lookup_server_connector/lib/BackgroundJobs/RetryJob.php(150): OC\Security\IdentityProof\Signer->sign('lookupserver', Array, Object(OC\User\User))
#6 .../nextcloud/lib/public/BackgroundJob/Job.php(79): OCA\LookupServerConnector\BackgroundJobs\RetryJob->run(Array)
#7 .../nextcloud/apps/lookup_server_connector/lib/BackgroundJobs/RetryJob.php(113): OCP\BackgroundJob\Job->execute(Object(OC\BackgroundJob\JobList), Object(OC\Log))
#8 .../nextcloud/cron.php(151): OCA\LookupServerConnector\BackgroundJobs\RetryJob->execute(Object(OC\BackgroundJob\JobList), Object(OC\Log))
#9 {main}

Steps to reproduce

I don't know how to reproduce it at the moment but it seems to occur if notification should be send-out.

Expected behavior

Execute background job without running into an error condition.

Installation method

Community Manual installation with Archive

Operating system

No response

PHP engine version

PHP 8.0

Web server

Apache (supported)

Database engine version

MariaDB

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

Updated from a minor version (ex. 22.2.3 to 22.2.4)

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

## Environment
#### Server Configuration
OS: Linux 5.15.64
Web server: Apache2 2.4.53
Database: MariaDB 10.3.35
PHP version: 8.0.20
Nextcloud version: 24.0.5

#### Client Configuration
Browser: Mozilla Firefox 104.0.2
Operating system: Windows 10

List of activated Apps

Enabled:
  - accessibility: 1.10.0
  - activity: 2.16.0
  - admin_audit: 1.14.0
  - announcementcenter: 6.3.1
  - apporder: 0.15.0
  - audioplayer: 3.3.0
  - bookmarks: 11.0.1
  - bruteforcesettings: 2.4.0
  - calendar: 3.5.0
  - circles: 24.0.1
  - cloud_federation_api: 1.7.0
  - comments: 1.14.0
  - contacts: 4.2.0
  - contactsinteraction: 1.5.0
  - dav: 1.22.0
  - event_update_notification: 1.5.0
  - external: 4.0.0
  - federatedfilesharing: 1.14.0
  - federation: 1.14.0
  - files: 1.19.0
  - files_accesscontrol: 1.14.1
  - files_antivirus: 3.3.1
  - files_automatedtagging: 1.14.0
  - files_downloadactivity: 1.13.0
  - files_external: 1.16.1
  - files_pdfviewer: 2.5.0
  - files_photospheres: 1.24.1
  - files_retention: 1.13.2
  - 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
  - firstrunwizard: 2.13.0
  - groupfolders: 12.0.1
  - guests: 2.2.0
  - impersonate: 1.11.0
  - logreader: 2.9.0
  - lookup_server_connector: 1.12.0
  - mail: 1.13.8
  - maps: 0.2.1
  - metadata: 0.16.0
  - news: 18.1.1
  - nextcloud_announcements: 1.13.0
  - notes: 4.5.1
  - notifications: 2.12.1
  - notify_push: 0.4.0
  - oauth2: 1.12.0
  - password_policy: 1.14.0
  - photos: 1.6.0
  - previewgenerator: 5.0.0
  - privacy: 1.8.0
  - provisioning_api: 1.14.0
  - serverinfo: 1.14.0
  - settings: 1.6.0
  - sharebymail: 1.14.0
  - smb_test: 0.3.4
  - spreed: 14.0.4
  - suspicious_login: 4.2.0
  - systemtags: 1.14.0
  - tasks: 0.14.4
  - text: 3.5.1
  - theming: 1.15.0
  - twofactor_backupcodes: 1.13.0
  - twofactor_gateway: 0.20.0
  - twofactor_totp: 6.4.0
  - twofactor_webauthn: 0.3.1
  - unsplash: 1.2.5
  - updatenotification: 1.14.0
  - user_status: 1.4.0
  - viewer: 1.8.0
  - workflow_script: 1.9.0
  - workflowengine: 2.6.0
Disabled:
  - dashboard: 7.0.0
  - encryption
  - epubreader: 1.4.7
  - files_trackdownloads: 1.11.0
  - recommendations: 0.6.0
  - support: 1.4.0
  - survey_client: 1.1.0
  - twofactor_admin: 3.2.0
  - user_ldap
  - weather_status: 1.0.0

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

.

Additional info

.

Start with lib/private/Security/Crypto.php:97 and trace the identity-proof flow through IdentityProof/Manager.php, IdentityProof/Signer.php, and lookup_server_connector/lib/BackgroundJobs/RetryJob.php. Run the cron background job using the reported PHP 8 setup and determine why the key is empty or the HMAC fails; done means the job completes without either error.