headless sounds to make alot of sense for a headless Ubuntu Server without GUI. But I’m not sure since it states there is no support for applications with GUI and I’m not sure if this is a problem to seafile Server or not…
according to my server-6.3.4 Elasticsearch 2.4.5 is also used.
looking at this SupportMatrix: Support Matrix | Elastic only version 8 of jre supports Elasticsearch 2.4.x