When using the advanced search in the OPAC, it will fail when the top field is left blank (or ignore subsequent fields if there is a blank field above them). It does appear to work in the intranet, however.
Hi Benjamin, I believe this was fixed in newer versions (you can check on the publicly available demo installations https://koha-community.org/demo/). Please consider updating to a supported version if you are still using 19.11 as mentioned above.