[Bug]: After Upgrade to NC33 Image Previews no longer accessible "CustomMessage":"Class \"OC\\Preview\\Storage\\Root\" not found"

[multiCheffe]

⚠️ 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

Hi,
I just update through updater.phar to newest NC33 from the latest NC32.
After the update all image previews are ignored. The preview folder in appdata_xxxx is still accessible and holds the required images, but they will not be used anymore.
As I have multiple Ks of images, the on-the-fly generation now takes ages and slows down the server to a near stop.

I used the memories and journeys app that were marked as "unsupported" for the 33 version and had them activated nevertheless. After facing the problems I deactivated them again which didn't change or help.

Steps to reproduce

Expected behavior

Using preview data that is already present

Nextcloud Server version

33

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.4

Web server

Apache (supported)

Database engine version

MariaDB

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

Upgraded to a MAJOR version (ex. 31 to 32)

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": {
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": {
            "0": "localhost",
            "1": "192.168.33.10",
            "5": "my.domain.here"
        },
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "33.0.0.16",
        "overwrite.cli.url": "https:\/\/my.domain.here",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "memcache.local": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "port": 6379,
            "timeout": 0
        },
        "tempdirectory": "\/var\/www\/nextcloud\/data\/tmp",
        "mail_smtpmode": "smtp",
        "mail_smtpauthtype": "LOGIN",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "jpeg_quality": 60,
        "overwriteprotocol": "https",
        "theme": "",
        "maintenance": false,
        "loglevel": 3,
        "mail_sendmailmode": "smtp",
        "mail_smtpsecure": "tls",
        "mail_smtpauth": 1,
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "587",
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "preview_max_x": 2048,
        "preview_max_y": 2048,
        "app_install_overwrite": [
            "calendar",
            "maps",
            "memories",
            "passwords",
            "journeys"
        ],
        "updater.release.channel": "stable",
        "default_phone_region": "DE",
        "maintenance_window_start": 1,
        "memories.db.triggers.fcu": true,
        "memories.exiftool": "\/var\/www\/nextcloud\/apps\/memories\/bin-ext\/exiftool-amd64-glibc",
        "memories.vod.path": "\/var\/www\/nextcloud\/apps\/memories\/bin-ext\/go-vod-amd64",
        "memories.gis_type": 1,
        "memories.vod.ffmpeg": "\/usr\/bin\/ffmpeg",
        "memories.vod.ffprobe": "\/usr\/bin\/ffprobe",
        "memories.vod.disable": false,
        "enabledPreviewProviders": [
            "OC\\Preview\\Image",
            "OC\\Preview\\Movie"
        ],
        "memories.vod.vaapi": true,
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "defaultapp": "dashboard"
    }
}

List of activated Apps

Enabled:
  - activity: 6.0.0-dev.0
  - bruteforcesettings: 6.0.0-dev.0
  - circles: 33.0.0
  - cloud_federation_api: 1.17.0
  - comments: 1.23.0
  - contacts: 8.3.3
  - contactsinteraction: 1.14.1
  - dashboard: 7.13.0
  - dav: 1.36.0
  - federatedfilesharing: 1.23.0
  - federation: 1.23.0
  - files: 2.5.0
  - files_downloadlimit: 5.1.0-dev.0
  - files_external: 1.25.1
  - files_pdfviewer: 6.0.0-dev.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-dev.0
  - logreader: 6.0.0
  - lookup_server_connector: 1.21.0
  - nextcloud_announcements: 5.0.0
  - notes: 4.13.0
  - notifications: 6.0.0
  - notify_push: 1.3.0
  - oauth2: 1.21.0
  - password_policy: 5.0.0-dev.0
  - passwords: 2025.12.21
  - photos: 6.0.0-dev.0
  - previewgenerator: 5.12.1
  - privacy: 5.0.0-dev.0
  - profile: 1.2.0
  - provisioning_api: 1.23.0
  - recognize: 11.0.1
  - recommendations: 6.0.0-dev.0
  - related_resources: 4.0.0-dev.0
  - serverinfo: 5.0.0-dev.0
  - settings: 1.16.0
  - sharebymail: 1.23.0
  - spreed: 23.0.1
  - support: 5.0.0
  - survey_client: 5.0.0-dev.0
  - systemtags: 1.23.0
  - tasks: 0.17.1
  - text: 7.0.0-dev.3
  - theming: 2.8.0
  - twofactor_backupcodes: 1.22.0
  - twofactor_totp: 15.0.0-dev.0
  - updatenotification: 1.23.0
  - user_status: 1.13.0
  - viewer: 6.0.0-dev.0
  - weather_status: 1.13.0
  - webhook_listeners: 1.5.0
  - workflowengine: 2.15.0
Disabled:
  - admin_audit: 1.23.0
  - app_api: 33.0.0 (installed 32.0.0)
  - encryption: 2.21.0
  - files_antivirus: 6.1.0 (installed 6.1.0)
  - journeys: 0.20.2 (installed 0.20.2)
  - memories: 7.8.2 (installed 7.8.2)
  - suspicious_login: 11.0.0-dev.0
  - twofactor_nextcloud_notification: 7.0.0
  - user_ldap: 1.24.0

Nextcloud Signing status

No errors have been found.

Nextcloud Logs

Additional info

No response

[Langelus]

My server was basically useless after the upgrade, visiting any page or app that had thumbnails locked it up for 30 seconds or more.

Quick fix for me was to delete the preview files and clear all entries in oc_filecache that related to previews and now it's working again at rebuilding the previews (no lockups anymore). Since it was over 2.2M Rows for me I took it in batches with a bash oneliner

while true; do r=$(mariadb -N -B nextcloud -e "DELETE FROM oc_filecache WHERE path LIKE 'appdata_[randomstring]/preview/%' LIMIT 60000; SELECT ROW_COUNT();"); echo "Deleted $r rows"; [ "$r" -eq 0 ] && break; done

[drunk-moe]

One thing I noticed is slow queries kept getting hammered with oc_filecache lookups; I created an index (I tried running occ files:scan-app-data before this and while it can take a while, instead of 20 minutes it was taking over an hour so I killed nextcloud and started looking into my mariadb).

I'm still testing but for now I created an index

USE nextcloud;
CREATE index phash_lookup ON oc_filecache (path_hash);

and now I'm not seeing anything logged in my slow queries files. I haven't tried to regenerate thumbnails or anything like that yet, but I will if this still has issues.

[SystemKeeper]

The index usage should be fixed by #58403 already, without creating a dedicated index for the hash. Maybe you can try it.

[drunk-moe]

@SystemKeeper can confirm, this seems to have helped a lot. both running pre-generate or using the UI is way quicker. I did let files:scan-app-data run for 6 or so hours before I killed it because I got tired of waiting for everything to migrate from the filecache table to the new previews table. Still going to keep this index for now because it seems to vastly improve stability but I might delete it later to check to see if it starts having long running queries again.

Thanks so much for your help!

[multiCheffe]

Thank you all for your recommendations.
Deleting all oc_filecache entries regarding appdata_XXXXXXX/preview did solve the problem. Now I let the preview-generator app do its work once again (will take days because of ~10M pics) but the system is at last reusable.
However, this should not have happend and the fix that is mentioned in #58403 should already be in the version I downloaded and installed, but still it did not work as expected.

[drunk-moe]

I think it was backported but not actually included in the 33 release so you'd have to manually patch it following the directions here. I can't confirm that because I do not work on/for nextcloud but that's my understanding of how backports work. Either way, glad it is working now for you.