On several jobs Jenkins lets me download artifacts only on recent builds, older builds give a 404 on their artifacts. 

If I browse the jobs's directory with the "artifact"-URL, e.g. "https://jenkins.domain.loc/job/exampleProject/1934/artifact/" the directory listing is shown empty on builds (with the warning "Symlinks are hidden") that give a 404 and non-empty on builds with downloadable artifacts.

However, there's no build discarder set and the files are still available in the "jenkins/jobs/<build-nr>" directory on the Jenkins master. Actually, the directory structure is the same on builds that have downloadable artifacts and builds that don't.

The files are also still linked as "Build Artifacts" on the build's page, e.g. "https://jenkins.domain.loc/job/exampleProject/1934/"  (but this links give 404, as said).

There's also no warning or error message in the Jenkins log when accessing the links.

In addition, there are also builds affected by this problem that did not run in months and that once had working artifacts URLs which now also give 404. 

I'm a bit lost here how to diagnose that further. The problem did also exist in the LTS version, we updated to the most recent Jenkins but that did not fix the issue.