[Bug]: Grid view fails to open folders on external storage (Local / SFTP) while list view works (NC 31.0.9)

⚠️ 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

Actual behavior

• In grid view: clicking any folder shows

  "Bei diesem externen Speicher ist ein Fehler aufgetreten. Möchtest du die Mount-Punkt-Konfiguration auf der Einstellungsseite überprüfen?"

• No XHR requests are fired in the browser dev tools when clicking a folder.
• In list view the same folders work fine.
• This happens with Local and also with SFTP (root) backends.
• Tested with previews disabled ('enabledPreviewProviders' => [] in config.php).
• nextcloud.log at loglevel 0 does not show any new entry when the error occurs.

Additional information

• Nextcloud version: 31.0.9
• Tested browsers: Firefox (private mode, cleared cache), same result.
• Admin user, external storage configured in Administration (not personal).
• Reproducible with different external storage backends (Local, SFTP).
• Console errors observed before (with RAW previewer, then disabled):
  • "The handler doesn't have a valid component"
  • "A setting with the same name is already registered"

Conclusion

This appears to be a frontend bug in the Files app grid view / viewer handling.
External storage is accessible and works in list view, but grid view folder navigation breaks entirely.

Steps attempted

• Disabled previews, disabled extra viewer apps (pdfviewer, richdocuments, etc.)
• Cleared appdata_xxx/js/ and repaired via occ, no effect.
• Tested with Local and SFTP backend → identical behavior.
• Verified filesystem permissions: www-data has full access, also reproducible with SFTP as root.

Impact

Grid view cannot be used for external storage mounts. List view remains functional.

Steps to reproduce

1. Create an external storage mount in Nextcloud 31.0.9:
   • Backend: Local (path e.g. /home/...Fotos/)
   • or Backend: SFTP (even tested with root access to the same folder)
   • RW permissions, previews disabled, applicable to one user/group.
2. Open Files app in list view → folders can be opened normally.
3. Switch to grid view and try to open a folder inside the external storage.

Expected behavior

Folders on external storage should open in grid view just like in list view.

Nextcloud Server version

30

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

List of activated Apps

Nextcloud Signing status

Nextcloud Logs

Additional info

No response

Start with the Files app grid view and reproduce the failure using both Local and SFTP external-storage mounts, comparing it with list view while watching browser requests and console output. Trace the grid-view folder navigation and viewer-handling path; done means folders on external storage open in grid view without the reported error.