On Thu, Nov 2, 2023 at 8:27 AM Peter Radisson <radisson97@web.de> wrote:
I am willing the help to debug that if somebody is telling me where to look.

Try running rebuildLocalisationCache.php or rebuildmessages.php.

In general for broken local installs, asking on IRC (e.g. #mediawiki) is best because people can help you debug the issue in realtime. There's also https://www.mediawiki.org/wiki/Project:Support_desk which tends to be more lively than this mailing list.