cannot run explorer, server ...

in the download there are the files objectdb.jar ... including the version number. It is impossible to run explorer or server using the file name objectdb-2.9.4_01.jar or after copying using objectdb.jar. The message that appears is like

Ursache: java.lang.ClassNotFoundException: com.objectdb-2.9.4_01.Server
(base) MacBook-Pro-2:bin hgz$ java com.objectdb.Server start
Fehler: Hauptklasse com.objectdb.Server konnte nicht gefunden oder geladen werden
Ursache: java.lang.ClassNotFoundException: com.objectdb.Server

Thank you for this report. Could you please provide the full command that you are trying to run.
For example, the following commands do work on Ubuntu 25.04:

java -jar explorer-2.9.4_01.jar

java -jar objectdb-2.9.4_01.jar start

Did you try this or something else? Is there a difference between version 2.9.4 and 2.9.4_01?

on downloading the latest version again it works (no glue why there was that problem)