Deck 1.15.2: Attachments not displayed in Nextcloud 31 (TypeError in frontend)

Bug: Deck Attachments not displayed after migration to Nextcloud 31

Description

After migrating from a working Nextcloud instance, Deck attachments (files and folders) are no longer displayed in cards, although they are correctly present in the database.

Environment

• Nextcloud Version: 31.0.9
• Deck Version: 1.15.2
• PHP Version: 8.3.6
• Database: PostgreSQL 16.10
• Webserver: Nginx 1.24.0
• OS: Ubuntu 24.04.3 LTS

Steps to Reproduce

1. Performed migration from Nextcloud 30.0.2 → 31.0.9
2. Deck data (Boards, Cards, Shares) fully migrated
3. Open card with attachments

Expected Behavior

Attachments (files/folders) are displayed in the "Attachments" tab of the card

Actual Behavior

• Paperclip icons are displayed (count is correct)
• But no attachments are rendered
• Buttons to add new attachments are missing
• JavaScript errors in browser console:

  TypeError: Cannot read properties of undefined (reading 'toLowerCase')
  TypeError: Cannot read properties of null (reading 'hasOwner')
  

Database Analysis

oc_share table (correct):

SELECT * FROM oc_share WHERE share_type = 12 AND share_with = '1419';

Result: 2 shares present, both with valid file_source IDs

oc_deck_attachment table:

SELECT * FROM oc_deck_attachment WHERE card_id = 1419;

Result: Empty (0 entries)

oc_filecache (files exist):

SELECT fc.fileid, fc.path FROM oc_filecache fc 
JOIN oc_share s ON s.file_source = fc.fileid 
WHERE s.share_type = 12 AND s.share_with = '1419';

Result: All files exist in filesystem

API Response

The API (/apps/deck/...attachments) returns correct data:

[
  {"id": 21351, "cardId": 1419, "type": "file", "data": "TESTORDNER"},
  {"id": 21383, "cardId": 1419, "type": "file", "data": "WS-Projektliste.xlsx"}
]

Additional Information

• Problem occurs only after migration
• Source system (NC 30.0.2, Deck 1.15.2): Works correctly
• New system (NC 31.0.9, Deck 1.15.2): Does not work
• Both systems have identical configuration
• Cache cleared multiple times (Redis, Browser, OPcache)
• Problem persists in incognito mode

Suspected Cause

Incompatibility between Deck 1.15.2 and Nextcloud 31.x - JavaScript code cannot properly process the API response.

Logs

No relevant errors in:

• /var/www/nextcloud/data/nextcloud.log
• /var/log/nginx/error.log

Only JavaScript errors in browser console.

Workaround Needed

Is there a way to make Deck 1.15.2 compatible with Nextcloud 31, or is an update to Deck 1.16+ required?

Start with the /apps/deck/...attachments API response and reproduce the browser console errors while opening a migrated card in the Attachments tab. Compare how the frontend processes the returned attachment objects on Nextcloud 30 and 31. Done means migrated attachments render, add-attachment controls appear, and both reported TypeErrors are gone.