JOSM cannot connect through a HTTP proxy requiring credentials

[Gingko]

What steps will reproduce the problem?

1. Open Edit -> Settings… (or whatever name it has in English, I use French version)

2. Select "Proxy".

3. Configure a private HTTP Proxy, filling host, port, user and password.

4. Try to do anything requiring network (displaying map, object data request, opening this ticket page, etc).

What is the expected result?

Be able to use JOSM more or less normally through my proxy.

What happens instead?

A window containing a message telling that data transfer is impossible, with details :
Unable to tunnel through proxy. Proxy returns "HTTP/1.1 407 Proxy Authentication Required".
Screenshot provided.

Please provide any additional information below. Attach a screenshot if possible.

I own the proxy I wanted to connect through.
It is an Apache proxy running on a Linux Debian server (I have several like that).
So I can read its logs.
The logging reports many connect attempts with 407 errors.
But nothing indicating that any login with credentials have been attempted, no matter if these credentials are good or bad.

I can provide dedicated testing credentials to my proxy if needed, please write me privately for that.

Revision:19412
Build-Date:2025-06-03 14:12:44

Identification: JOSM/1.5 (19412 fr) Windows 10 64-Bit
OS Build number: Windows 10 Pro 22H2 (19045)
Memory Usage: 432 MB / 24352 MB (258 MB allocated, but free)
Java version: 21.0.7+6-LTS, Azul Systems, Inc., OpenJDK 64-Bit Server VM
Look and Feel: javax.swing.plaf.metal.MetalLookAndFeel
Screen: \Display0 1920x1200x32bpp@75Hz (scaling 1.00×1.00) \Display1 1920x1200x32bpp@59Hz (scaling 1.00×1.00) \Display2 1200x1920x32bpp@59Hz (scaling 1.00×1.00) \Display3 1280x1024x32bpp@60Hz (scaling 1.00×1.00)
Maximum Screen Size: 1920×1920
Best cursor sizes: 16×16→32×32, 32×32→32×32
System property file.encoding: UTF-8
System property sun.jnu.encoding: Cp1252
Locale info: fr_FR
Numbers with default locale: 1234567890 -> 1234567890
VM arguments: [-Djpackage.app-version=1.5.19412, -XX:MaxRAMPercentage=75.0, --add-modules=java.scripting,java.sql,javafx.controls,javafx.media,javafx.swing,javafx.web, --add-exports=java.base/sun.security.action=ALL-UNNAMED, --add-exports=java.desktop/com.sun.imageio.plugins.jpeg=ALL-UNNAMED, --add-exports=java.desktop/com.sun.imageio.spi=ALL-UNNAMED, --add-opens=java.base/java.lang=ALL-UNNAMED, --add-opens=java.base/java.nio=ALL-UNNAMED, --add-opens=java.base/jdk.internal.loader=ALL-UNNAMED, --add-opens=java.base/jdk.internal.ref=ALL-UNNAMED, --add-opens=java.desktop/javax.imageio.spi=ALL-UNNAMED, --add-opens=java.desktop/javax.swing.text.html=ALL-UNNAMED, --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED, -Djpackage.app-path=%UserProfile%\AppData\Local\JOSM\JOSM.exe]

Plugins:
+ apache-commons (36438)
+ busstoptool (1.1.0)
+ fastdriveway (v1.0.1)
+ jaxb (36438)
+ mapathoner (v1.7.1)
+ mapwithai (839)
+ pmtiles (36438)
+ utilsplugin2 (36438)

Map paint styles:
- https://josm.openstreetmap.de/josmfile?page=Styles/MapWithAI&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/Admin_Boundaries&zip=1
- https://josm.openstreetmap.de/josmfile?page=Rules/IncompleteObjectWarnings&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/PublicTransport&zip=1
+ https://josm.openstreetmap.de/josmfile?page=Styles/PublicTransportV2&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/Waterways&zip=1

Last errors/warnings:
- 00000.465 W: extended font config - overriding 'filename.Mongolian_Baiti=monbaiti.ttf' with 'MONBAITI.TTF'
- 00001.132 E: java.security.KeyStoreException: Windows-ROOT not found. Cause : java.security.NoSuchAlgorithmException: Windows-ROOT KeyStore not available
- 00030.506 W: java.io.IOException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 407 Proxy Authentication Required"
- 00030.507 W: java.io.IOException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 407 Proxy Authentication Required"
- 00030.639 W: java.io.IOException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 407 Proxy Authentication Required"
- 00030.639 W: Already here java.io.IOException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 407 Proxy Authentication Required"
- 00030.640 W: java.io.IOException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 407 Proxy Authentication Required"
- 00036.218 E: java.io.IOException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 407 Proxy Authentication Required"
- 00036.253 E: org.openstreetmap.josm.io.OsmTransferException: Impossible de joindre le serveur. Veuillez vérifier votre connexion Internet.. Cause : java.io.IOException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 407 Proxy Authentication Required"
- 00036.261 E: Exception I/O - <html>Impossible dʼenvoyer ou de recevoir des données depuis<br>'https://overpass-api.de/api/interpreter'<br>dû à un problème de transfert de données.<br>Details (non traduit): Unable to tunnel through proxy. Proxy returns "HTTP/1.1 407 Proxy Authentication Required"</html>

[Gingko]

Screenshot of the error message