Cannot complile JOSM using Eclipse

[GerdP]

I don't know since when, but my Eclipse installation no longer worked. So I started with a fresh installation using
​https://www.eclipse.org/downloads/download.php?file=/technology/epp/downloads/release/2025-09/R/eclipse-java-2025-09-R-win32-x86_64.zip&mirror_id=1285
and an empty workspace.
I followed the steps described in the wiki https://josm.openstreetmap.de/wiki/DevelopersGuide/Compiling
After importing the project c:\josm\core the build is started automatically and ends with a popup showing

Errors occurred during the build.
Errors running builder 'Java Builder' on project 'josm'.
Internal Error compiling /josm/src/org/openstreetmap/josm/gui/widgets/PopupMenuButton.java

The log window shows two more problems.

The problem remains when I do "Maven -> Update Project... -> Select All -> OK"