Media download error in Element-Desktop 1.11.86: “! error downloading image matrix”

[Dunduk99]

Steps to reproduce

1.	Install Element-Desktop version 1.11.84, 1.11.85, or 1.11.86 on a Linux system Debian 12.
2.	Log in to a self-hosted Matrix server configured with Nginx as a reverse proxy.
3.	Join a room or start a direct message.
4.	Send or receive an image file.
5.	Try to download or view the image.

Outcome: The image does not load, and the client displays the error:
! error downloading image matrix.
Logs from Element-Desktop console:

Access to fetch at https://example.com/_matrix/client/v1/media/download/room22/iRuxgrAfspaiDcWBllvLKKlo?allow_redirect=true 
from origin  vector://vector' has been blocked by CORS policy: No  Access-Control-Allow-Origin  header is present on the requested resource.

Unable to download attachment to decrypt it: DownloadError: Failed to fetch
    at a.downloadSource (Media.ts:144:27)
    at d (DecryptFile.ts:49:38)
    at r.getFn (MediaEventHelper.ts:88:24)
    at get value (LazyValue.ts:40:26)
    at r.getFn (MediaEventHelper.ts:66:51)
    at get value (LazyValue.ts:40:26)
    at O.downloadImage (MImageBody.tsx:255:62)
    at O.componentDidMount (MImageBody.tsx:266:18)
    at yI (react-dom.production.min.js:260:217)
    at bI (react-dom.production.min.js:259:46)

GET https://example.com/_matrix/client/v1/media/download/room22/iRuxgrAfspaiDcWBllvLKKlo?allow_redirect=true net::ERR_FAILED

Access to fetch at https://example.com/_matrix/media/v3/download/room22/ZGZwEudLGsfsefOeeaMpuKVW?allow_redirect=true
from origin vector://vector has been blocked by CORS policy: No Access-Control-Allow-Origin header is present on the requested resource.

My Matrix server is hosted on Debian 12 with Nginx as a reverse proxy.
The server is non-federated and used for private purposes only.
My Nginx configuration is slightly different from the standard setup. Here’s the configuration:

    location /custompath/ {
        proxy_pass http://localhost:8008/;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_pass_request_headers on;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
    }

The issue started with Element-Desktop 1.11.84. Everything worked flawlessly in version 1.11.83 with the same server configuration.
It seems that something changed in how media URLs or paths are handled in Element-Desktop. Could stricter path handling or enforced absolute links have been introduced?

Outcome

What did you expect?

• Media files (e.g., photos or images) should download and display correctly in the Element-Desktop client, as they did in version 1.11.83.
• Users should be able to receive, decrypt, and view images in rooms or direct messages without errors.

What happened instead?

• The client displays the error ! error downloading image matrix.
• Media files (only photos or images) fail to download or decrypt.
• The browser console logs show CORS-related issues, specifically the absence of Access-Control-Allow-Origin headers.
• The issue only occurs on Element-Desktop versions 1.11.84, 1.11.85, and 1.11.86. Earlier versions work without issues.
• iOS clients using the same server configuration can download and view media files (photos/images) without errors.

Operating system

Linux Debian Bookworm

Application version

1.11.86

How did you install the app?

apt get install element-desktop

Homeserver

matrix-synapse 1.119.0

Will you send logs?

Yes