Sorry, I found the correct setting. This is solved.
I upgraded to JabRef 5.13 (JabRef 5.13–2024-04-01–6bdcf63, Linux 5.14.21-150400.24.100-default amd64, Java 21.0.2, JavaFX 22+30) and encountered a strange error, which I never had before:
When I include citations from different libraries in one document (Libre Office 7.6.2.1), I can not create the Reference list, because entries are only found from one of the libraries, even though I have all libraries open in JabRef. For entries from other libaries, I get the error “Unresolved”.
Is this a known bug - or did I miss to set some prefernces, which were default in older versions?