[Koha] z39.50 search does not return results

Hey there, I have another issue after moving Koha to a new server: The Z39.50/SRU search is no longer returning results (/cgi-bin/koha/cataloguing/z3950_search.pl). Settings are the same as on the old system, where everything works fine. There are no indications of errors in the logs. Can anyone give me an idea, where to look for a solution of the problem? Is there an option to make Koha more talkative to get an idea what causes this problem? Thanks for any help, Hauke