10.9-Unstable, Unstable Plugin Repository not working

[pixel24]

Hello everyone,
Following the documentation, I removed the Stable Plugin Repository and added the Plugin-Unstable repository, then restarted the server.
Subsequently, I cleared the browser cache and logged in as an admin again. The catalog is completely empty. In the log file, I find:

Code:

Mär 27 17:37:20 media02 jellyfin[848]: System.Net.Http.HttpRequestException: Response status code does not indicate success: 500 (Proxy Failure).
Mär 27 17:37:20 media02 jellyfin[848]:    at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()
Mär 27 17:37:20 media02 jellyfin[848]:    at System.Net.Http.Json.HttpClientJsonExtensions.<FromJsonAsyncCore>g__Core|12_0[TValue,TJsonOptions](HttpClient client, Task`1 responseTask, Boolean usingResponseHeadersRead, CancellationTokenSource linkedCTS, Func`4 deserializeMethod, TJsonOptions jsonOptions, CancellationToken cancellationToken)
Mär 27 17:37:20 media02 jellyfin[848]:    at Emby.Server.Implementations.Updates.InstallationManager.GetPackages(String manifestName, String manifest, Boolean filterIncompatible, CancellationToken cancellationToken)
Mär 27 17:37:29 media02 jellyfin[848]: [17:37:29] [INF] WS 192.168.83.141 request
Mär 27 17:37:48 media02 jellyfin[848]: [17:37:48] [ERR] An error occurred while accessing the plugin manifest: https://repo.jellyfin.org/files/plugin-unstable/manifest.json
Mär 27 17:37:48 media02 jellyfin[848]: System.Net.Http.HttpRequestException: Response status code does not indicate success: 500 (Proxy Failure).
Mär 27 17:37:48 media02 jellyfin[848]:    at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()
Mär 27 17:37:48 media02 jellyfin[848]:    at System.Net.Http.Json.HttpClientJsonExtensions.<FromJsonAsyncCore>g__Core|12_0[TValue,TJsonOptions](HttpClient client, Task`1 responseTask, Boolean usingResponseHeadersRead, CancellationTokenSource linkedCTS, Func`4 deserializeMethod, TJsonOptions jsonOptions, CancellationToken cancellationToken)
...

++ Test Environment ++

+ Server +
OS: Debian 15.5 / latest patch level
Jellyfin 10.9 / Build 2024032505
NGINX: 1.22.1-9 / Configured according to Jellyfin documentation

+ Client +
Linux Mint 21.3 Virgini / Firefox 124.0.1
Official Android Client 2.6.0